Galatians 3:19, 24

Why, then, the Law [what was its purpose]? It was added [after the promise to Abraham, to reveal to people their guilt] because of transgressions [that is, to make people conscious of the sinfulness of sin], and [the Law] was ordained through angels and delivered to Israel by the hand of a mediator [Moses, the mediator between God and Israel, to be in effect] until the Seed would come to whom the promise had been made.

with the result that the Law has become our tutor and our disciplinarian to guide us to Christ, so that we may be justified [that is, declared free of the guilt of sin and its penalty, and placed in right standing with God] by faith.