19 Why, then, was the law given at all? It was added because of transgressions until the Seed to whom the promise referred had come. The law was given through angels and entrusted to a mediator.
(Read Galatians 3:19-22)
If that promise was enough for salvation, wherefore then serveth the law? The Israelites, though chosen to be God's peculiar people, were sinners as well as others. The law was not intended to discover a way of justification, different from that made known by the promise, but to lead men to see their need of 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 by the ministry of angels, and the hand of a mediator, even Moses. Hence the law could not be designed to set aside the promise. A mediator, as the very term signifies, is a friend that comes between two parties, and is not to act merely with and for one of them. The great design of the law was, that the promise by faith of Jesus Christ, might be given to those that believe; that, being convinced of their guilt, and the insufficiency of the law to effect a righteousness for them, they might be persuaded to believe on 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 to all, should be contrary to the gospel of Christ. It tends every way to promote it.
Matthew Henry's Commentary on Deuteronomy 33:2
Commentary on Deuteronomy 33:1-5
(Read Deuteronomy 33:1-5)
To all his precepts, warnings, and prophecies, Moses added a solemn blessing. He begins with a description of the glorious appearances of God, in giving the law. His law works like fire. If received, it is melting, warming, purifying, and burns up the dross of corruption; if rejected, it hardens, sears, pains, and destroys. The Holy Spirit came down in cloven tongues, as of fire; for the gospel also is a fiery law. The law of God written in the heart, is a certain proof of the love of God shed abroad there: we must reckon His law one of the gifts of his grace.