And if you belong to Christ, then you are Abraham’s descendants, heirs according to promise (Galatians 3:29). 

This means that Gentiles, through faith in Christ, are grafted into the covenantal blessings of Israel. But this inclusion does not nullify the original promises to the physical descendants of Abraham, Isaac, and Jacob. Instead, it reveals the Messiah as the hinge upon which those promises swing open to the nations.  - Joel Richardson, Abraham Covenant, Paul, and Galatians 3:16