There is a concept in the New Testament that is largely misunderstood by many teachers, and that is the concept called “the fullness of the Gentiles.” Paul refers to this concept in several places in the book of Romans. The first is Rom 11.12 where he says “Now if their (Israel) transgression (false step by not believing in Yeshua) be riches for the world and their failure be riches for the Gentiles, how much more will their fulfillment be.” The second one is Rom 11.25 where he says ” For I do not want you, brethren, to be uninformed (through lack of correct doctrine) of this mystery, lest you be wise in your own estimation, that a partial hardening has happened to Israel until the fullness of the Gentiles has come in.”
Now, Paul is not referring to the Gentiles with this term but he is referring to Gen 48.19 in a prophecy about Ephraim: “But his father refused and said ‘I know, my son, I know, he (Manasseh) will also be great. However, his younger brother (Ephraim) shall be greater than he and his descendants shall become the fullness of the Gentiles (“m’loh ha goyim” in Hebrew).” Jacob says that Ephraim (whose name means “fruitful”), who would later be known as the ten northern tribes of Israel, would be scattered among the nations (Isa 7.8; Amos 9.9; Jer 34.17; Hos 9.16-17; Ezek 37.21). The ten northern tribes were cut off and divorced by the Lord because of their unbelief. Later the two remaining tribes were also cut off and scattered among the nations.
So, all twelve tribes have been scattered among the Gentiles. Rom 11 describes what happened and how the Lord will save Israel (all twelve tribes) and bring them back in faith (emunah). In Rom 11.25 Paul says that a hardening has happened to Israel (all twelve tribes) until the “fullness of the Gentiles (“m’loh ha goyim”=the twelve tribes scattered among the Nations/Gentiles) comes in” (by faith in Yeshua). Then in v 26 he says that “thus (after this happens) all Israel (all twelve tribes) will be saved” and he quotes Isa 59.20 (see also Rom 9.27; 11.5; Jer 31.1-10; 34.17; Jer 50 17-20; Ezek 37.21).
Paul is telling the Gentile believers in Rome that what has happened to Israel was predicted. They have only “stumbled” but not fallen (Rom 11.11) and it is temporary. God will save a remnant of all twelve tribes and put them together again (Ezek 37). The term “fullness of the Gentiles” does not mean when all the Gentiles who will believe comes to faith in Yeshua, it means those among Israel who have been scattered among the Gentiles who come back to God through faith in Yeshua. The story of the Prodigal Son in Luke 15.11-32 is telling the same story and it is based on Jer 31.1-20 where this story is called “The Merciful Father” in Hebrew thought.