Paul therefore concludeth with this sentence: “they which are of faith, are the children of Abraham.” Corporal birth, or carnal seed maketh not the children of Abraham before God. As though he would say, there is none before God who is accounted as the child of this Abraham (whom God hath chosen, and made righteous by faith) through carnal generation: but such children must be given him before God, as he was a father. But he was a father of faith, was justified and pleased God, not because he could beget children after the flesh, not because he had circumcision and the law, but because he believed God. He therefore that will be a child of the believing Abraham, must also himself believe, or else he is not a child of the elect and justified Abraham, but only of the begetting Abraham, which is nothing else but a man conceived, born, and wrapped in sin, without faith, and therefore condemned.This boasting then, “we are the seed of Abraham,” is to no purpose.
Martin Luther, Kregel, Galatians 3:7, p. 135.