There were 12 tribes until Joseph's two boys received the birthright blessings from Israel, adopting them as his own. [Genesis 48:5-6. Ephraim and Mannasseh now had equal status as "Sons of Israel".Now we have thirteen. Later the tribe of Levi became a priestly tribe and divided itself among the other tribes. This was the tribe of Moses and Aaron. Now we are back to twelve.
1 Kings 11:11-13 [So the lord said to Solomon, "Since this is your attitude and you have not kept my covenant and my decrees, which I commanded you, I will most certainly tear the kingdom away from you and give it to one of your subordinates. Nevertheless, for the sake of David your Father, I will not do it during your lifetime. I will tear it out of the hand of your son. Yet I will not tear the whole kingdom from him, but will him one tribe for the sake of David my servant and for the sake of Jerusalem, which I have chosen.]
Now we have two kingdoms....one of the North and one of the South. The Northern kingdom(ten tribes) kept the name of Israel and the Southern kingdom(two tribes), composed of Judah and Benjamin was called Judah which was later shortened to Jews. The Northern kingdom was carried off into captivity by the Assyrians in about 721 B.C. and became known as the "Lost Tribes" because they never came back. Judah was later taken captive by the Babylonians but they returned to Jerusalem as chronicled by Ezra and Nehemiah.
The dude on the throne of David would be Jesus.
Jeremiah 52:10-11, [There at Riblah the king of Babylon slaughtered the sons of Zedekiah before his eyes ; He also killed all the officials of Judah. Then he put out Zedekiah's eyes, bound him with bronze shackles and took him to Babylon where he put him in prison to the day of his death.]
Notice the lack of descendants from the line of David all of a sudden! Who was now on the throne?....Jesus would not be born for another 600 years. Remember God's promise from Jeremiah 33. There will always be a descendant of David to sit on the throne continually. Where was that throne and who was now sitting on it?
1 Kings 9:4-7
4 And if thou wilt walk before me, as David thy father walked, in integrity of heart, and in uprightness, to do according to all that I have commanded thee, and wilt keep my statutes and my judgments:
5 Then I will establish the throne of thy kingdom upon Israel for ever, as I promised to David thy father, saying, There shall not fail thee a man upon the throne of Israel.
6 But if ye shall at all turn from following me, ye or your children, and will not keep my commandments and my statutes which I have set before you, but go and serve other gods, and worship them:
7 Then will I cut off Israel out of the land which I have given them; and this house, which I have hallowed for my name, will I cast out of my sight; and Israel shall be a proverb and a byword among all people:
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Ezekiel 37:21-24
21 And say unto them, Thus saith the Lord GOD; Behold, I will take the children of Israel from among the heathen, whither they be gone, and will gather them on every side, and bring them into their own land:
22 And I will make them one nation in the land upon the mountains of Israel; and one king shall be king to them all: and they shall be no more two nations, neither shall they be divided into two kingdoms any more at all:
23 Neither shall they defile themselves any more with their idols, nor with their detestable things, nor with any of their transgressions: but I will save them out of all their dwellingplaces, wherein they have sinned, and will cleanse them: so shall they be my people, and I will be their God.
24 And David my servant shall be king over them; and they all shall have one shepherd: they shall also walk in my judgments, and observe my statutes, and do them.