Is it individual believers or the whole Gentile system that is grafted into Israel?
The whole Gentile system is grafted into that which was originally Israel, not individual believers as the primary focus. Believers are certainly part of it, but the grafting concerns the corporate Gentile system, which is compared to an old, no-good olive tree grafted into the beautiful original root, Israel.
Les explains that the grafting is not about individual believers as the ones grafted in, although they are certainly part of it. He clarifies that the focus is the whole Gentile system, which is grafted into that which was originally Israel. The Gentiles are likened to an old, no-good olive tree grafted into the beautiful original root, Israel. This rests on God's promise to Abraham in Genesis 12:3, that all families of the earth would be blessed—a promise never abrogated. Israel enjoyed the fatness of the root of Abraham until Paul turned to the Gentiles.
And if some of the branches be broken off, and thou, being a wild olive tree, wert graffed in among them, and with them partakest of the root and fatness of the olive tree;