Römer 4:3

For what saith the, Scripture? Abraham believed God, and it--his faith. was counted to him for righteousness-- (Gen 15:6). Romish expositors and Arminian Protestants make this to mean that God accepted Abraham's act of believing as a substitute for complete obedience. But this is at variance with the whole spirit and letter of the apostle's teaching. Throughout this whole argument, faith is set in direct opposition to works, in the matter of justification--and even in Rom 4:4-5. The meaning, therefore, cannot possibly be that the mere act of believing--which is as much a work as any other piece of commanded duty (Joh 6:29; Jo1 3:23) --was counted to Abraham for all obedience. The meaning plainly is that Abraham believed in the promises which embraced Christ (Gen 12:3; Gen 15:5, &c.), as we believe in Christ Himself; and in both cases, faith is merely the instrument that puts us in possession of the blessing gratuitously bestowed.

The great gospel doctrine of justification by faith without the works of the law was so very contrary to the notions the Jews had learnt from those that sat in Moses' chair, that it would hardly go down with them; and therefore the apostle insists very largely upon it, and labours much in the confirmation and illustration of it. He had before proved it by reason and argument, now in this chapter he proves it by example, which in some places serves for confirmation as well as illustration. The example he pitches upon is that of Abraham, whom he chooses to mention because the Jews gloried much in their relation to Abraham, put it in the first rank of their external privileges that they were Abraham's seed, and truly they had Abraham for their father. Therefore this instance was likely to be more taking and convincing to the Jews than any other. His argument stands thus: "All that are saved are justified in the same way as Abraham was; but Abraham was justified by faith, and not by works; therefore all that are saved are so justified;" for it would easily be acknowledged that Abraham was the father of the faithful. Now this is an argument, not only pari - from an equal case, as they say, but fortiori - from a stronger case. If Abraham, a man so famous for works, so eminent in holiness and obedience, was nevertheless justified by faith only, and not by those works, how much less can any other, especially any of those that spring from him, and come so far short of him in works, set up for a justification by their own works? And it proves likewise, ex abundanti - the more abundantly, as some observe, that we are not justified, no not by those good works which flow from faith, as the matter of our righteousness; for such were Abraham's works, and are we better than he? The who…

4:3 Paul quotes Gen 15:6. In response to God’s promise that he would have descendants as numerous as the stars in the sky, Abraham believed God. It was faith that established Abraham’s relationship with God—not works (Rom 4:3-8), circumcision (4:9-12), the law (4:13-17), or the number of his descendants (4:18-21).