Romans 4:2

‘For if Abraham was reckoned as in the right by works, he has that in which to glory, but not towards God.'

But Paul reacts strongly against the suggestion that Abraham was reckoned as righteous by God because of his works. He declares that if Abraham really was reckoned as in the right by works, as the unbelieving Jews claimed, (he is making a concession, notice the ‘if') it could only be in the eyes of men. He would then have a cause of boasting before men. But, Paul stresses, he would not have a grounds of boasting before God. For God requires, not partial, but total obedience. He will agree that in the eyes of men Abraham might well be highly esteemed and be seen as better than most men, so that he could glory/boast before men. But he will not for one moment concede that he had any grounds for boasting before God. This is a position which he now demonstrates from Scripture, which must be the final arbiter (as both Jew and Christian would agree).

Note how this argument reflects Romans 3:27 ‘Where then is the glorying? It is excluded. By what manner of law? Of works? No, but by a law of faith.'