Galatians 3:29 explains that, “if you belong to Christ, then you are Abraham’s descendants, heirs according to promise.” The passage leads some to conclude that those who are in Christ are ultimately grafted into Israel, and receive the covenant blessings promised to Israel. Some might call this supersessionism (replacement theology), others assert this is simply God adding to His people, but not necessarily replacing one with another. Regardless of which perspective is in view, there are significant problems with the understanding that based on Galatians 3:29 Israel and the church are indistinct. In fact there is a readily identifiable logical fallacy in play: If all who belong to Christ are Abraham’s descendants, then all Abraham’s descendants belong to Christ. This is the error of affirming the consequent. We cannot argue for the indistinctness of Israel and the church based on Galatians 3:29 for the same reason we cannot argue that because all cars have wheels, everything with wheels must be cars.
But besides the logical issue, there is exegetical data supporting the distinctness of Israel and the church based on Abraham’s fatherhood. Notice in Romans 4 Abraham is identified as a father in several ways. First, in 4:1, he is a “forefather according to the flesh.” When compared with 4:12, we observe this to be a clear reference to his fathering the Jewish people in the physical sense only – meaning that these sons of Abraham are sons in no other way but physical (based on Abraham’s descendants being named through Isaac, as in Genesis 21:12, and Jacob, based on Genesis 50:24). As these are not described as having the faith of Abraham, we might describe them as Jewish unbelievers, or at least Jews in general, who are not specially identified by their faith.
Romans 4:11 identifies Abraham as “the father of all who believe without being circumcised.” These are not physically related, but are still his children in that they have the same kind of faith as he did (compare Genesis 15:6 with Romans 4:11). These are Gentile believers.
Romans 4:12 describes another kind of progeny of Abraham. He is “the father of circumcision to those who not only are of the circumcision, but who also follow in the steps of the faith of our father Abraham…” These are both physically related and related in the sense of following in his faith. These are believing Jews. They are of the circumcision, and they have faith resulting in righteousness.
Broadly speaking, Romans 4 cites three descendants of Abraham: (1) the physical only descendant (the unbelieving Jew), (2) the spiritual only descendant (the believing Gentile), and (3) the physical and spiritual descendant (the believing Jew).
Notice in Galatians 3:9 that those of faith are blessed with Abraham, related to 3:6, which quotes Genesis 15:6, and related to 3:8, a passage referencing that the Gentiles would be justified by faith. Also observe that 3:16 references the promises (plural) that were spoken to Abraham, but that 3:29 describes Abraham’s descendants as heirs according to “a promise” – singular, not plural. Those in Christ – and specifically the Gentiles who have faith (3:8) are heirs according to a promise. They are the “you” of 3:29 (note 5:2, the “you” are the uncircumcised). They do not receive all the promises (plural) – the first six promises God made to Abraham (Genesis 12:2-3a), but they do receive the blessing of the seventh promise (Genesis 12:3b) – that all the families or nations of the earth would be blessed through Abraham.
In light of these observations, two of the three kinds of descendants of Abraham are believing (one is the remnant of Israel – Jews who believe, the other is the believing Gentile), but another kind isn’t believing. These are simply related to Abraham through the fleshly lineage. And only one of the three (believing Jews) will receive the promised covenant blessings. Consequently, if we suggest that all the descendants of Abraham are in the same lump and all receive the same promises and blessings, then we are failing to recognize clear distinctions between the three groups described in Romans 4. Certainly, in the body of Christ there is no distinction between Jew and Greek (Galatians 3:28), but that is in the scope of the body of Christ, and that indistinctness does not invalidate the distinctness that was present before the body of Christ was formed, nor after its completion.
Israel and the church remain distinct, and receive different blessings. Certainly, believing Jews and Gentiles in the body of Christ, or the church age, enjoy the same blessings, still, even in this age, ethnic Israel is not the church, nor does the church somehow become ethnic Israel. Consequently, when God makes promises to ethnic Israel, He intends to keep those promises faithfully to those to whom He made those promises. Anything less would be a violation of His word.