Galatians 3:19-20

If such was not the function of the Law — if it had no power to modify the promise — what was its true function? It was a sort of measure of police. Its object was to deal with transgressions. It was also a temporary measure, of force only until it should be superseded by the coming of the Messiah. Unlike the promise, too, it was a contract. It was given by a mediator — that is, a person acting between two parties. Two parties were involved, with rigid conditions binding them both. On the other hand, the promise was given unconditionally by the sole act of God.

In stating the true function of the Law, the Apostle brings out its inferiority to the promise in four respects. (1) It dealt with sins, not with holiness; (2) it was temporary and transitory; (3) it was given, not directly, but indirectly, through the double mediation of the angels and of Moses; (4) it was conditional, and not like the promise, unconditional. It depended upon the fallible action of man, and not only upon the infallible word of God.