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Because the law worketh wrath--has nothing to give to those who break is but condemnation and vengeance. for where there is no law, there is no transgression--It is just the law that makes transgression, in the case of those who break it; nor can the one exist without the other.

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…

4:15 The only way to avoid breaking the law is to have no law to break! (literally where there is no law, neither is there transgression): Paul always uses the word “transgression” to denote disobedience of a clear commandment of God (see also 2:23; 5:14; Gal 3:19; 1 Tim 2:14). Transgression only exists where the law exists, which is why the law always brings punishment. The law that God gave to the Israelites specified requirements in great detail, which made the people more accountable for sin than before. So when they inevitably disobeyed the law, God brought more severe punishment upon them.