Salvation Through the Ages, Chap. 6, The Age of Law
From Israel to Jesus
God allowed Israel to be enslaved in Egypt for 400 years before giving them a written Law to obey. No more were people left to be on their honor to obey God, no more would there be any excuses. They would now have specific written regulations to abide by. Predictably, though, Israel repeatedly strayed from God and suffered repeated judgments, until finally God had to drive them out of their land. Only after several centuries were they allowed to return, and then only in very small numbers, a mere remnant. It was to this remnant that Jesus came, only to be conspired against and crucified.
This age is without doubt the one most cited as the foundation and basis of Christianity. Yet it is all about Israel as a nation, very sharply defined and limited. As noted already, the Promise was never affected nor superseded by the Law; they are two separate entities.
This point is magnified by the apostle Paul in several passages, most notably Galatians 3. In that passage he states that the purpose of Jesus' sacrifice was that "the blessing given to Abraham might come to the Gentiles through Christ Jesus." The law had nothing to do with it. Like a formal "last will and testament", the death of the testator (God incarnate) released the Jews from it, and the dead are not liable for any contracts. And of course the Gentiles were never under it at all.1 In fact, in Rom. 2:14 Paul said that the Gentiles only were responsible for obeying their consciences, "the law within."
The natural question then arises, of what purpose was the law? Paul answers, to bring the people of Israel safely to the timely arrival of the promised Seed, like a custodian or guardian seeing to the proper raising of a child. It was never supposed to be a permanent "nanny", nor to keep the "child" from coming of age. The law served the Promise, not the other way around.