Albert Barnes Commentary Romans 4:2

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

For if Abraham, etc. This is the answer of the apostle. If Abraham was justified on the ground of his own merits, he would have reason to boast or to claim praise. He might regard himself as the author of it and take the praise for himself (Romans 4:4). Therefore, the inquiry was whether, in the account of Abraham's justification, any such statement giving a reason for self-confidence and boasting was to be found.

But not before God. In the sight of God. That is, in his recorded judgment, he had no ground of boasting on account of works. To show this, the apostle appeals at once to the Scriptures, to demonstrate that there was no such record indicating that Abraham could boast he was justified by his works. Since God judges rightly in all cases, it follows that Abraham had no just ground of boasting and, consequently, that he was not justified by his own works. The meaning of this verse is well expressed by Calvin: "If Abraham was justified by his works, he might boast of his own merits. But he has no ground of boasting before God. Therefore he was not justified by works."