Matthew Henry Commentary Romans 3:27-31

"Where then is the glorying? It is excluded. By what manner of law? of works? Nay: but by a law of faith. We reckon therefore that a man is justified by faith apart from the works of the law. Or is God [the God] of Jews only? is he not [the God] of Gentiles also? Yea, of Gentiles also: if so be that God is one, and he shall justify the circumcision by faith, and the uncircumcision through faith. Do we then make the law of none effect through faith? God forbid: nay, we establish the law." — Romans 3:27-31 (ASV)

God wills that the great work of the justification and salvation of sinners be carried on from first to last in such a way as to exclude boasting. Now, if we were saved by our own works, boasting would not be excluded. But the way of justification by faith forever excludes boasting.

Yet believers are not left to be lawless; faith is a law, it is a working grace wherever it is genuine. By faith—which in this context is not an act of obedience or a good work, but rather forms the relationship between Christ and the sinner—it becomes fitting that the believer should be pardoned and justified for the Savior's sake. Consequently, the unbeliever, who is not united or related to Him in this way, remains under condemnation.

The law is still of use to convince us of what is past, and to direct us for the future. Though we cannot be saved by it as a covenant, yet we acknowledge and submit to it as a rule in the hand of the Mediator.