What was God doing with Israel through the Law for 1,500 years?
For 1,500 years God used the Law as a schoolmaster to train, teach, and prepare Israel, hoping to bring them to maturity as believing Jews. That purpose was to prepare the nation to be vessels and instruments God could use to reach the Gentile world.
Les explains that for 1,500 years Israel was under the Law, which Paul identifies specifically as the Moral Law, the Ten Commandments, functioning as a schoolmaster to bring them to Christ. It was a system by which God was training them, teaching them, and hopefully bringing them to the place of mature, understanding, believing Jews. That was God's hope, though it did not happen. The purpose of the Law was to prepare the nation of Israel for a great opportunity: to be vessels and instruments He could use to bring in the masses of the Gentile world.