Israel (who is of course more commonly called Jacob) had twelve sons: Reuben, Dan, Naphtali, Gad, Asher, Issachar, Zebulun, Joseph, Levi, Simeon and Benjamin, (Genesis 35:21-26) and at least one daughter, Dinah. These twelve are called brothers although they are in some cases half brothers, having between them four different mothers. The descendents of Jacob through these twelve sons are the people of Israel, and were divided into twelve tribes each descended from one of these sons.
The descendents of Judah later took the name Jews, so the Jews are and always were a subset of the Israelites. No problem so far.
(Well, we might wonder about whether Dinah had any descendents or sisters, particularly as Jewishness is inherited from the mother not the father, but let's not do that here. And Jacob later adopted two of his grandsons, explicitly giving them rank with his sons, with consequences we can't escape, but more on that below.)
Jacob and all his descendents travelled to Egypt to escape a famine in Canaan, where they had been living as aliens. Some centuries later the now far more numerous Israelites under Moses travelled from Egypt, where they had been reduced to slavery. Moses and his brother Aaron were from the Tribe of Levi. After forty years as nomads, the Isrealites conquered the "Promised Land" of Canaan under Moses' successor Joshua, of the Tribe of Ephraim. No, Ephraim is not on the list above, and we'll return to this. Joshua allocated land in the conquered and about to be conquered territory to each of the twelve tribes, sort of, and we'll return to this too.
This now landed nation of Israel was in turn conquered and exiled several times over the following centuries, and after one such exile, only the tribe of Judah returned, so from that time onwards the only remaining Israelites were the Jews, as stated in 2 Kings 17:18-19: So the Lord was very angry with Israel and removed them from his presence. Only the tribe of Judah was left, and even Judah did not keep the commands of the Lord their God. They followed the practices Israel had introduced.(NIV)
Still no great problem. The Jews are still a subset of the Israelites, just no longer a proper subset. So in that sense the two terms became synonyms, so much so that the religion of Israel is now called Judaism, again after the tribe of Judah.
And the tribes that didn't return are called the Lost Ten Tribes. Hang on. Twelve minus ten is two, not one. So, if only one tribe, Judah, is left, where did the other tribe go?
Oh, it gets worse...! There are many lists of the names of the Lost Ten Tribes, some more commonly quoted than others, and the number of names on these lists varies from eight to thirteen (at least). And there are in all at least seventeen different tribe names in the various lists in the Bible.
The Sons of Jacob are listed in Genesis 29:31–30:24 and 35:16-20 (the list on this occasion is drawn out because Benjamin is born so much later than the others); Genesis 35:22-26; Genesis 46:8-27; and Exodus 1:1-5: Reuben, Simeon, Levi, Judah, Dan, Naphtali, Gad, Asher, Issachar, Zebulun, Joseph, and Benjamin. Only the order differs between these four lists.
Numbers 1:5-15 explicitly names twelve tribes or armies, and names a commander for each of these twelve tribes. But this list does not include Levi or Joseph, but instead the two Sons of Joseph, Manasseh and Ephraim. And Manasseh and Ephraim both now have a claim to being Sons of Israel, as in Genesis 48 they are both quite explicitly adopted by Jacob as sons, to rank equally with his natural sons: your two sons born to you in Egypt before I came to you here will be reckoned as mine; Ephraim and Manasseh will be mine, just as Reuben and Simeon are mine. Any children born to you after them will be yours; in the territory they inherit they will be reckoned under the names of their brothers. (NIV)
The other ten are the same as the previous lists of the Sons of Israel. Numbers 1:20-54 lists twelve families, and the list is almost the same as Numbers 1:5-15 except that the sons of Joseph, namely, of the sons of Ephraim is one entry, and Manasseh is listed without qualification. The omission of Levi is also explained here. Numbers 2:3-29 lists the same twelve tribes or armies, this time divided into four camps. For each tribe a commander is named, and for each camp a leading tribe is named. The West camp is led by Ephraim and contains Manasseh, and there is now no mention of Joseph. The Levites are again mentioned as exempt from military service. The same list appears in Numbers 7:1-88 where the commanders of the twelve tribes each present an offering. Much the same list appears in the marching orders of Numbers 10:11-28, of four camps, twelve tribes and their leaders, except that the Levites are not mentioned as a group but instead are divided into three families each with special duties and not included in any other tribe.
Numbers 13:4-15 nominates a man from each of their fathers’ tribes, twelve in all, to spy out the land, and omits Levi completely but again lists both Ephraim and Manasseh. But this time it’s Manasseh that is associated with Joseph, and Ephraim is listed without qualification. Then in Numbers 26:5-50 the same twelve appear, this time further divided into families, and both Ephraim and Manasseh are listed as Sons of Joseph.
This list is unchanged in Numbers 34:13-28, the allocation of land, except that Manasseh is itself now divided into two half tribes, and again there is no mention of Joseph at all. Counting Manasseh as one, we still have twelve.
Then in Deuteronomy it gets very interesting. Deuteronomy 27:12-13 goes back to the twelve Sons of Israel from Genesis; Levi and Joseph are back in, Manasseh and Ephraim are out. Then Moses blesses these tribes in Deuteronomy 33:1-29, except now he leaves out Simeon. No explanation is given, but Manasseh and Ephraim are both now mentioned, not as tribes but as Sons of Joseph, and no other tribe gets its finer divisions or families mentioned here.
Joshua 13–19 is again an allocation of land, so Levi is out and Manasseh and Ephraim are back in, with no mention of Joseph. Simeon is back in. Manasseh is again divided into two half tribes, the Eastern portion having received their land, the Western still to occupy it. This same list is repeated in Joshua 21, where the Levites also receive land but are not regarded as a tribe.
There’s another list in Judges 5:12-22, the Song of Deborah. Levi, Joseph, Judah and Simeon are all omitted, and Gad and Manasseh are possibly not mentioned. Ephraim and Benjamin are praised, as are Makir (which some commentators say is Manasseh, or may be half of it), Zebulun, Issachar and Naphtali, while Reuben, Gilead (which may be the other half of Manasseh, but some commentators say is Gad), Dan and Asher are criticised for not joining the battle. So at most ten tribes are listed here, including possibly all three branches of Joseph, and at least three, and possibly four, of the Sons of Israel are not represented at all.
Then 1 Chronicles 2:1-2 starts out promisingly. It’s the familiar list of Jacob’s twelve sons. There’s a minor worry perhaps in that the detailed genealogies in 1 Chronicles 2–7 then leave out Zebulun, it doesn’t actually say this is a complete family tree but that seems implied. But 1 Chronicles 12:24-38 really comes unstuck when listing David’s armies. Joseph is out, but Levi is in with an army of 4,600 men, as are Ephraim and the two half-tribes of Manasseh, so even counting Manasseh as one tribe we still get thirteen, and all of them including Levi providing an army, unlike in Exodus where Levi was excused from military service and this is given as the reason not to count them as one of the twelve. 1 Chronicles 27:16-22 fixes this sort of by leaving out both Gad and Asher, and is unique in including a tribe of Aaron, who are earlier explicitly listed as part of Levi (1 Chronicles 12:26-28). 1 Chronicles 27:16-22 also identifies the two half tribes as Manasseh in Gilead and just plain Manasseh, which may tie in with the names Gilead and Makir in Judges. This list has again thirteen members including Levi and the two half tribes, so comparing to the thirteen in 1 Chronicles 12:24-38 it's tempting to say that the "half tribes" are here each being considered as full members of the twelve tribes.
The last Old Testament writer to list the tribes was Ezekiel. In Ezekiel 48:1-29 twelve tribes are listed, returning to the Genesis list but leaving out Joseph in favour of Ephraim and Manasseh, and not really saying whether Levi is a tribe or not, it would make thirteen if so. Then in Ezekiel 48:30-34 the gates named for the Tribes of Israel are listed, and it’s the Genesis list exactly, with Joseph and Levi in and Ephraim and Manasseh out again.
The only list of the tribes in the New Testament is Revelation 7:5-8. It is unique in omitting Dan. It includes Levi, Joseph and Manasseh, but not Ephraim. Revelation 7:4 may say that these are all the tribes of Israel but not all commentators agree on this reading.
In summary, there are several different lists, some of them possibly incomplete. The tribes (and half tribes) they contain are Reuben, Simeon, Levi, Judah, Dan, Naphtali, Gad, Asher, Issachar, Zebulun, Joseph, Benjamin, Ephraim, Manasseh, East Manasseh, West Manasseh, Gilead, Machir and Aaron, making nineteen in all, or seventeen if you leave out the half tribes.
At first glance some of these appear to be a subset of another, and so both should not be in the same list. Ephraim and Manasseh are both parts of Joseph, and Aaron part of Levi. But several such seemingly impossible combinations do appear in the lists in the Bible.
But no one list has more than thirteen tribes, and when there are thirteen one of them is always Levi. Levi does however also appear in some lists that seem complete and contain only twelve.
Seven tribes are easily accounted for. Reuben, Dan, Naphtali, Gad, Asher, Issachar and Zebulun are similar and relatively simple stories.
After King Solomon died, the Kingdom of Israel again split in two, but this time the split was not the simple power struggle between the tribes of Benjamin (King Saul) and Judah (King David) and their supporters. This time the split was geographic, and soon also came to be on theological lines.
The lands of these seven tribes plus the tribe(s) of Joseph were contiguous and occupied the northern part of the original Kingdom of Israel that had been ruled by Saul, David and Solomon. These tribes together with some of the Levites rebelled and set up what is often called the Northern Kingdom, with its own king, Jeroboam of the tribe of Ephraim, and with its capital at Samaria.
This kingdom existed for two centuries only, during which time it had nineteen kings from no fewer than nine royal houses and from various tribes, including Issachar and possibly Gad. Some of these kings ruled only a few days.
And this Northern Kingdom was and is also, confusingly, called Israel to distinguish it from the Southern Kingdom of Judah. This is obviously a very politically charged question, both at the time of the split and to this day. In many ways the name Israel would have been more appropriately applied to the Southern Kingdom, which continued to be ruled by the same dynasty as was in power in the original Kingdom of Israel just prior to the split, and was ruled from the same capital, Jerusalem. But most of the tribes of Israel were in the Northern Kingdom, in fact even by this time only the tribe of Judah was really a force in the Southern Kingdom. So the Southern Kingdom took the name Judah, and the rebels took the name Israel, temporarily as it turned out.
The Northern Kingdom fell to invaders, and some of the people were carried off to exile in many stages and over many years and by several conquerors. More seriously, those that remained intermarried with other peoples, adopted some of the religious practices of these other peoples, and ceased to think of themselves as Israelites. Some of them took the name Samaritans, after the name of their capital Samaria which was also the name of the region around it. For this they were held in contempt by the Southern Kingdom, who always regarded themselves as the true heirs of Israel, and after this none of the tribes that formed the Northern Kingdom ever regained their identity as a tribe of Israel. The people of the Southern Kingdom didn’t want them. We can’t say for sure whether they ever sought this recognition, the only possible record if they did is in Ezra/Nehemiah, which was written by those of the Southern Kingdom.
Most lists of the tribes of Israel (whether in the Bible or not) list Reuben, Dan, Naphtali, Gad, Asher, Issachar and Zebulun as seven of the Twelve Tribes, and also, if they make such a distinction, as seven of the Lost Ten Tribes. Again, no great problem so far.
Some commentaries do reject Reuben as even being one of the Twelve, because of the incident in Genesis 35:22 While Israel was living in that region, Reuben went in and slept with his father’s concubine Bilhah, and Israel heard of it (NIV). But the tribe appears in every list in the Bible, both before and after this incident.
And it's the tribes that are lost, not their members. It was loss of their relationship to Israel and eventually of their whole cultural identity, rather than genocide.
The tribe or house or family of Joseph was part of the Northern Kingdom and shared the fate of the seven tribes that were unambiguously part of it. The only question is, how many of these tribes were there? Estimates vary from one to three.
In Genesis 48:5 we read that in recognition of Joseph saving the whole family of Jacob by bringing them to Egypt, Jacob adopted Joseph's two sons, Manasseh and Ephraim, and from then on Joseph’s descendents are most often considered to form two or even three separate tribes, these tribes belonging to the Family or House of Joseph. As already mentioned, Joshua is described as of the tribe of Ephraim, not the tribe of Joseph. This makes up the number of tribes to twelve when Levi or Simeon or Benjamin is left out. Manesseh and Ephraim are sometimes referred to as half tribes by commentators, but in the Bible the only half tribes described as such are Eastern Manasseh and Western Manasseh. Wherever Ephraim or Manasseh in its entirety is mentioned, they are accorded the status of full tribes, not of half tribes.
Revelation 7:4-8 leaves out Dan and compensates for this by including both Joseph and Manasseh while leaving out Ephraim.
Manasseh was the elder brother, but in Genesis 48:17-20 Jacob gave Ephraim the blessing of the firstborn, foretelling that he would be greater than his elder brother, and when the kingdom split, the first King of the Northern Kingdom came from the tribe of Ephraim, giving some credence to this. But then Revelation leaves Ephraim out but leaves Manasseh in. Splitting Manasseh on occasions helps to make up the number of tribes to twelve, and the number of Lost Tribes to ten.
Judah is the only tribe that is unambiguously unlost.
David and Solomon, the kings of the golden age of the original, united Kingdom of Israel, with its capital in Jerusalem, were from the tribe of Judah. After the split, the Southern Kingdom was known as Judah, but its capital remained at Jerusalem in the territory of Benjamin. All of its kings came from the tribe of Judah.
And even before the split, there had been a tendency to regard Judah as separate from Israel. 1 Samuel 11:8 is the first place this distinction is made, and it’s repeated in many places from then on, many of them long before the split into North and South.
Later members of the tribe of Judah took comfort from 2 Kings 17:18, which states that God rejected the tribes of the Northern Kingdom, and only Judah was left. However, the context here is of God rejecting all of Israel, Judah included.
The Kingdom of Judah fell some time after the Northern Kingdom, but unlike the Northern Kingdom it was eventually rebuilt. But the interesting thing is, while those who returned to rebuild Jerusalem were the descendents of the exiled people of the Kingdom of Judah, they saw themselves as rebuilding not the fallen Southern Kingdom of Judah, but the earlier and far more glorious original, united Kingdom of Israel as ruled by David and Solomon (both of course of the Tribe of Judah).
In the Books of Ezra and Nehemiah, which describe the return from exile and were originally one book continuing the story of Chronicles, the terms People of Judah and People of Israel are used interchangeably to refer to the same group, in complete contrast to the usage in 1 Samuel and other passages up until the return. In most English translations of the Bible, chronologically the term Jew first appears in the Book of Esther, and then next in Ezra. (Note that although many Bibles print Esther after Ezra and Nehemiah, it's probably the other way around chronologically, and not terribly relevant here. Esther describes events during the exile, while Ezra and Nehemiah describe the end of the exile. The events described could also overlap.) From then on the terms Jew and Israelite are synonymous, including of course in the New Testament.
The leaders of the re-established Israel were from the tribes of Judah (such as Nehemiah), Benjamin, and Levi (such as Ezra); The Book of Ezra pointedly talks of the leaders only of Judah and Benjamin. The surrounding peoples, many if not all of them descended from other tribes of Israel, at first offered to support the project. This approach was rudely rejected, and from then on they opposed it, to the point that armed guards had to be posted to protect the workmen.
Although there would not explicitly be another King of Israel or of Judah, even as a tributary title, until Aristolubus I, it was Nehemiah who was in command. The neighbouring peoples accused him of plotting to become King of Judah (Nehemiah 6:7). For those members of other tribes who were part of the project, even members of Benjamin, taking part implied allegiance to the Tribe of Judah, and the implicit rejection of the idea that the tribes of the Northern Kingdom had any further claim to being part of Israel.
It's interesting to speculate as to whether both the nation they rebuilt and the modern nation of Israel would more properly be called Judah. Obviously, this distinction has enormous political consequences, both at the time of the rebuilding and to this day.
By the time of Jesus Christ, the three terms Hebrew, Israelite, and Jew (or at least, their equivalents in the then current languages, such as Greek, Latin, Aramaic and Hebrew but certainly not English) had become synonymous. The sign at Jesus' crucifixion read King of the Jews. And of course, the religion of the Hebrews was Judaism
But in any synagogue to this day, the worship is directed towards the God of Abraham, Isaac and Jacob; In other words, to the God of Israel, not just of Judah.
The Tribe of Levi served as priests, and received no major land allocation in Canaan, but received cities within the territories of each of the other tribes, and some pasture land for their herds and flocks adjacent to each of these cities. Whether they were one of the twelve tribes is ambiguous. They served as priests mainly in Jerusalem, but not exclusively, and those that were in the Northern Kingdom at the time of the split mostly sided with and emigrated to Judah, where they were regularly rostered for temple duty anyway. So while some members of Levi were carried off with the eight to ten tribes lost at the fall of the Northern Kingdom, most of them merged into Judah.
Many people with surnames such as Cohen or Levine trace their lineage to the Tribe of Levi, and many but not all of these also consider themselves to be Jewish. This continued existence of the tribe of Levi and also within it of descendents of Moses' brother Aaron is very important to those Jews who still hope to restore the temple and its system of worship, as certain functions within this system are reserved for Levites, and others for kohanim ("priests"). The kohanim are descendents of Aaron, and there are also other requirements on them that basically require a particularly pure Israelite lineage.
1 Chronicles 27:16-22 lists twelve officers in charge of "the twelve tribes of Israel", and it lists both Levi and Aaron as two of these tribes, and as noted above omits Gad and Asher. This may show the importance that had come to be given to the kohanim by this time, but it's the only place in the Bible that Aaron is listed as a separate tribe.
If we needed to pick another unlost tribe, Levi has the most credible claim. However Levi is generally listed as one of the Lost Ten Tribes, although the consensus is not as strong as for the seven simple cases and for Joseph. The problem is not so much whether they are one of the lost tribes, but whether they are one of the twelve Tribes of Israel at all.
The Tribe of Simeon is possibly the first to be lost, as it is missing from the list of tribes in Deuteronomy 33, however it survived long enough to be later allocated land in the southern part of Israel. Their allocation was an area of mainly desert completely surrounded by the territory of the tribe of Judah, and appears in some ways to be an afterthought. Judah had too much territory, so some of it was given to Simeon.
For whatever reason (and several have been given) it became unfashionable to be a Simeonite, so those with Simeonite fathers tended to associate instead with their mothers' tribes, and the identity of Simeon as a tribe was lost. It merged into the Tribe of Judah.
There is general agreement that Simeon is one of the Lost Ten Tribes, although again the consensus is not as strong as for the tribes of the Northern Kingdom. Simeon is mainly of interest because it suffered such a similar fate to that suffered much later by Benjamin, which is often regarded as the other unlost tribe along with Judah.
Benjamin was lost after Simeon and in similar fashion, however the reasons are more explicit and the process more drawn out. In Judges 19-21 the deliberate near-extermination of Benjamin by the other tribes is described. However they recovered sufficiently for the first King of Israel, Saul, to later come from the Tribe of Benjamin.
Possibly this is what led to their complete demise. Their history as a royal house started and ended with Saul. Following Saul's death, his only surviving son Ish-bosheth was proclaimed king, but never succeeded in uniting the kingdom. After two more years of civil war, David of the Tribe of Judah took the throne and moved his capital from Hebron, in the territory allocated to Judah, to Jerusalem, in the territory allocated to Benjamin, which was immediately to the north of the territory of Judah.
Benjamin was also a traditional ally of Ephraim, the ringleaders of the Northern Kingdom, so their later throwing their lot in with Judah rather than joining the rebellion by their northern neighbours was an interesting decision.
Benjamin went the way of Simeon and ceased to exist as a tribe and also merged into the Tribe of Judah. But as noted above, Levi is often listed as one of the Lost Ten Tribes. As there is general agreement that the seven of Reuben, Dan, Naphtali, Gad, Asher, Issachar and Zebulun are lost, and also Joseph and Simeon, including both Levi and Benjamin would make eleven lost tribes at least.
So Benjamin is often listed as one of the two unlost tribes, the other being of course Judah. This way of looking at it has the neat result that the two surviving tribes are the two that provided the two royal dynasties of the united Kingdom of Israel, although the Benjaminite dynasty of course only really lasted for one king, Saul. It is consistent with the books of Ezra and Nehemiah, which talk of the rebuilding of Jerusalem being undertaken by Judah and Benjamin. It also avoids the awkward question of how Paul of Tarsus, writing in what later became the New Testament, could claim to be an Israelite of the Tribe of Benjamin (Philippians 3:5, Romans 11:1).
So which is the second unlost tribe?
The traditional answer is Benjamin. But there's not a lot of difference between the status of Benjamin and that of Simeon, so if one is lost surely both are. And Simeon is generally accepted as lost. That leaves Levi.
Those living people who claim membership of Levi also generally claim membership of Judah. This is not unique to Levi. There are living people who have strong claims to belong to several, perhaps all, of the Tribes of Israel, but those who make these claims generally do so in the context of claiming to be Jewish. There is, for example, a Jewish community from Central Asia known as the Bukharan Jews many of whom bear the surname Issachar, which is not a traditional surname in any other community.
If what we're looking for is cultural identity rather than just living members, then after Judah, Levi rather than Benjamin is the tribe which has the best claim to being acknowledged as a continuing entity.
(What follows is not a particularly popular view. It's just the only one that makes very much sense. Democracy isn't always good at determining this. So don't blame me if you give this answer in a quiz show and get given wrong for it, or expect any support from the audience, or even from various encyclopedias etc..)
As stated above, the Twelve Tribes of Israel are Reuben, Dan, Naphtali, Gad, Asher, Issachar, Zebulun, Joseph, Levi, Simeon and Benjamin, each one founded by one of the Sons of Israel.
And ten of these twelve are indeed lost. Eight of these lost tribes, Reuben, Dan, Naphtali, Gad, Asher, Issachar, Zebulun, and Joseph, were lost when Israel In Samaria, the Northern Kingdom, fell. Simeon and Benjamin had already merged into Judah before this time, making ten that had been lost in all. But that's not necessarily what the Lost Ten Tribes means.
Ancient Israel was a military organisation, and the tribes were its upper level of structure. At the time Israel in Samaria fell, this structure still existed although it had been reorganised many times since the days of the original twelve tribes. The first such reorganisation was when Jacob himself authorised Joseph to divide into two, each to be accorded the status of a tribe. In later reorganisations, formations had merged or been disbanded, others had been split and others had been created anew from pieces of other formations. Sometimes tribe names that had disappeared in previous reorganisations reappeared in later ones. That's what the various lists tell us. The history of any large modern army reads similarly. Such changes are more frequent in wartime than in peacetime of course, but they are unceasing.
And again like any modern army, the Israelites operated under regulations. One of these regulations specified that at any time, the number of tribes or armies had to be exactly twelve. And it’s a very good regulation, one that some modern corporations have imitated, in order to achieve a management structure that is neither too flat nor too lofty. When he saw the need to split Joseph under two commanders, Moses complied with these regulations by also splitting the Levites into three smaller formations and making them all part of his headquarters.
So the Lost Ten Tribes are Reuben, Dan, Naphtali, Gad, Asher, Issachar, Zebulun, Ephraim, West Manasseh and East Manasseh, reflecting the organisation at the time of the fall of the Northern Kingdom, which was known at the time as Israel in Samaria or (confusingly) just as Israel. This nation is also entirely lost, for the present at least.
The other two tribes at this time seem to have been Judah and Benjamin, and these are the two that later returned. Although they had been deported from the kingdom of Judah, when they returned the (vassal) kingdom they established claimed to be the restoration of the earlier, united kingdom of Israel. But there was no attempt to restore the twelve tribe structure at this time, nor has there been since. For one thing, the Royal authority which had authorised the return probably wouldn't stretch that far; For another, Judah and Benjamin didn't want to be reconciled to the Samaritans, and the feeling was mutual; And for a third, it wasn't at all obvious exactly which other tribes to recognise even had they wanted to.
These two returned tribes subsequently again merged under the tribe name Judah, but they also reclaimed the national name of Israel at this time, finally making the names fully synonymous. This tribe/nation contained the remnants of Simeon and Benjamin, and the major portion of Levi. Levi is almost as lost as Simeon or Benjamin, as it also merged into Judah, and its status as one of the twelve was often ambiguous anyway.
Today members of the tribes of Benjamin, Levi, Issachar and Aaron still identify themselves as such, but there is no tribal identity. They also regard themselves primarily as Jews.
And so only Judah is left. And twelve minus ten equals one.