Charles Ellicott Commentary Romans 4:2

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

We know that he obtained justification. If that justification had been earned by his own works, it would then have been something to be proud of; it would be a point of pride he might fairly hold both toward men and toward God. For to men, he could point to the privileged position he had gained, and in the sight of God, he would be able to plead a certain merit of his own. But he does not have this merit. His justification was not earned but was bestowed upon him, not because of his works, but because of his faith. This is the express statement of Scripture. Therefore, it follows that although his privileged position in the sight of men remains, he has nothing to boast of before God.

But not before God.—This is an instance of the Apostle's rapid and eager dialectic. If the whole train of thought had been given, it would probably have proceeded much as described above; but the greater part of it is suppressed, and the Apostle strikes straight at the one point he intended to highlight. (Whatever there might be before men) there is no boasting before God.