If Abraham was indeed justified by works, he had something to boast about, but not before God.
2. For if Abraham, etc. This is an incomplete argument, Epicheirema; in Greek ἐπιχείρεμα, an attempted but an unfinished process of reasoning. It is not necessary to introduce this sort of syllogism, it being not the character of Scripture nor of any other writing to discuss matters in this form.
Here the apostle proves that Abraham was justified not by works, but by faith. Those that of all men contended most vigorously for a share in righteousness by the privileges they enjoyed, and the works they performed, were the Jews, and therefore he appeals to the case of Abraham their father, and puts his own name to the relation, being a Hebrew of the Hebrews: Abraham our father.
Commenting on Romans 4:1-8
For if Abraham were justified by works,.... That Abraham was not, and could not be justified by works, is clear from hence, that if this was his case, he hath whereof to glory; which will be allowed him before men, on account of his pious life and conversation: but not before God; who saw all the iniquity of his heart, and was privy to all...