So many false doctrines in here, it is mind-blowing. The tribulation is occurring NOW. The Millennium will occur in Heaven, after this Earth is completely destroyed by fire. But all of that is for other threads and other discussions.
Yes, both Israel and Judah were "divorced"/exiled and cast out because of their unbelief in the passage in Jeremiah. But that is irrelevant to the discussion. The nation was redeemed and brought back to Jerusalem before Jesus.
The Jews in the first century (other than the believing Jews) were cut off from the tree (Israel). Those who later believed were grafted back in. Those who did not believe were cast into the fire (Hell).
Gentiles were never part of the tree, but were able to be grafted in and become a part of the tree. The Church is that group of branches that have been grafted in. The Church is now part of the tree (Israel), thus it is proper to say that the Church is Israel (even though there is more to the tree than just the Church), just as, if you see a branch of an apple tree, you can say, "That is an apple tree."