This is one of the most confusing and debated passages of Scripture:
For I do not want you, brethren, to be uninformed 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; and thus all Israel will be saved.” Romans 11:25-26
Saint Paul put forth a “mystery”. A “partial hardening” has happened to ethnic Israel until the “fullness of the Gentiles”. At this point “all Israel” will be saved.
In other words, “Israel” plus “fullness of Gentiles” equals “all Israel”. So what is the “fullness of the Gentiles”?
I think that Saint Paul is referring to Genesis 48:17-19:
When Joseph saw that his father laid his right hand upon the head of Ephraim, it displeased him; and he took his father’s hand, to remove it from Ephraim’s head to Manasseh’s head.
And Joseph said to his father, “Not so, my father; for this one is the first-born; put your right hand upon his head.”
But his father [Jacob/Israel] refused, and said, “I know, my son, I know; he also shall become a people, and he also shall be great; nevertheless his younger brother shall be greater than he, and his descendants shall become a multitude of nations.”
That final line could be translated as “fullness of the Gentiles”. It foreshadows how Ephraim (synecdochical for the northern ten tribes) later “mixed itself” with the nations (Hosea 7:8).
Saint Paul indicates that the Israel/Ephraim’s identity is “mixed” with the Gentiles. In fulfillment of the prophecy of Jacob, the sons of of Ephraim have become the “fullness of the Gentiles” though in a less than noble way. Quite remarkable. For Paul, the only way to restore the lost remnant of Israel is to restore the Gentiles with whom they have mixed. This would explain how the Gentiles have benefited by the received curses of Israel.
It would also explain why Christ told the Apostles to proclaim the Gospel first in Jerusalem, then Judea, then Samaria, and then to the ends of the world. Samaria was the greatest concentration of “half-breed” northern Israelites. The evangelization of Samaria is the fulfillment of the restoration of “Israel” whether they be half-breeds or not. It is also noteworthy that both Peter and John specifically came to Samaria in order to impart the Holy Spirit via the sacrament of Confirmation (Acts 8:14-17).