Why was the Law given and how long was it to last?
The Law was given because of transgressions, added so the whole world became responsible and guilty before God, since no one could claim ignorance of sins like adultery, thieving, and coveting. It was not permanent; it was to last until the seed should come to whom the promise was made.
Les explains that before the Law, people’s wickedness could not be put to their account in the same way, because they were not breaking any written Law. He notes that you cannot break a Law if there is not one. But once the Law was given, the whole world became responsible—adultery, thieving, coveting, and all the way down through the ten were now clearly sin, so there was no excuse. Les then points to Galatians, where the Law was added because of transgressions until the seed should come to whom the promise was made, ordained by angels in the hand of a mediator. That fixes its duration: it was temporary, lasting until the promised seed came.
Wherefore then serveth the law? It was added because of transgressions, till the seed should come to whom the promise was made; and it was ordained by angels in the hand of a mediator.