For if you were cut from what is by nature a wild olive tree, and grafted, contrary to nature, into a cultivated olive tree, how much more will these, the natural branches, be grafted back into their own olive tree.
God’s people is like a big tree. In the OT He set aside a national people called Israel. Many of those people, at the time of Christ, rejected Jesus as Messiah, to their great detriment. Nationality is not, nor ever has it been, a marker of salvation from the Lord. If anyone, regardless of their biology, rejects Jesus, there is no hope for their salvation, they are, as it were, like a branch broken off of the tree of life which is called God’s people. But we, though not part of that national group, through our belief in Jesus, are grafted into the tree of life. We receive the same blessings and life that come from the genuine root, even though we are not by nature part of God’s national people. So what of national Israel? Well, they should understand the concept of Messiah even greater than we. How much easier should it be for them to believe. So what Paul is saying is this, there is no national people of God outside of Jesus, and those in Christ are part of one and the same people with everyone else regardless of background. Who your parents are has no bearing on whether or not you are saved, only Jesus.
