Therefore it is of faith that it might be according to grace, so that the promise might be sure to all the seed, not only to those who are of the law, but also to those who are of the faith of Abraham, who is the father of us all.
Ver. 16. There are two kinds of children of Abraham, to whom alone these promises are made; the one is according to the flesh, the other according to the spirit. The former of these had no more part in the promises made to him and his seed than the Gentiles, unless they imitated the fidelity and obedience of their father. (Calmet) — It is in this sense of spiritual father, that the [Catholic] priest at the altar, speaking in the name of the faithful, calls Abraham our patriarch. (Estius)