Matthew Henry Commentary Galatians 3:19-22

"What then is the law? It was added because of transgressions, till the seed should come to whom the promise hath been made; [and it was] ordained through angels by the hand of a mediator. Now a mediator is not [a mediator] of one; but God is one. Is the law then against the promises of God? God forbid: for if there had been a law given which could make alive, verily righteousness would have been of the law. But the scriptures shut up all things under sin, that the promise by faith in Jesus Christ might be given to them that believe." — Galatians 3:19-22 (ASV)

If that promise was enough for salvation, what purpose, then, did the law serve? The Israelites, though chosen to be God's special people, were sinners just like others. The law was not intended to reveal a way of justification different from that made known by the promise. Instead, its purpose was to lead people to see their need for the promise by showing the sinfulness of sin, and to point to Christ, through whom alone they could be pardoned and justified.

The promise was given by God Himself; the law was given through the ministry of angels and by the hand of a mediator, namely Moses. Therefore, the law could not have been designed to set aside the promise. A mediator, as the term itself signifies, is a friend who comes between two parties and is not to act merely with and for one of them.

The great purpose of the law was that the promise, received through faith in Jesus Christ, might be given to those who believe. This was so that, being convinced of their guilt and the insufficiency of the law to effect a righteousness for them, they might be persuaded to believe in Christ and so obtain the benefit of the promise. And it is not possible that the holy, just, and good law of God—the standard of duty for all—should be contrary to the gospel of Christ. It tends in every way to promote it.