A.T. Robertson Commentary Romans 4:2

"For if Abraham was justified by works, he hath whereof to glory; but not toward God." — Romans 4:2 (ASV)

The Scripture (η γραφη). Genesis 15:6.

Was justified by works (εξ εργων εδικαιωθη). Condition of first class, assumed as true for the sake of argument, though untrue in fact. The rabbis had a doctrine of the merits of Abraham who had a superfluity of credits to pass on to the Jews (Luke 3:8).

But not towards God (αλλ' ου προς θεον). Abraham deserved all the respect from men that came to him, but his relation to God was a different matter. He had there no ground of boasting at all.