Galatians 3:16 Now the promises were spoken to Abraham and to his seed. He does not say, “And to seeds,” as referring to many, but rather to one, “And to your seed,” that is, Christ. 17 What I am saying is this: the Law, which came four hundred and thirty years later, does not invalidate a covenant previously ratified by God, so as to nullify the promise. 18 For if the inheritance is based on law, it is no longer based on a promise; but God has granted it to Abraham by means of a promise. 19 Why the Law then? It was added because of transgressions, having been ordained through angels by the agency of a mediator, until the seed would come to whom the promise had been made. 20 Now a mediator is not for one party only; whereas God is only one. 21 Is the Law then contrary to the promises of God? May it never be! For if a law had been given which was able to impart life, then righteousness would indeed have been based on law. 22 But the Scripture has shut up everyone under sin, so that the promise by faith in Jesus Christ might be given to those who believe. 23 But before faith came, we were kept in custody under the law, being shut up to the faith which was later to be revealed. 24 Therefore the Law has become our tutor to lead us to Christ, so that we may be justified by faith.
Paul was fighting against the Judaizers who wanted to bring something into the Gospel and add one little something men were supposed to do in order to be saved. He tried to show them that 430 years before Moses led the children of Israel out of Egypt and God gave them the Law, the promises had already been made. Before the Law came to Israel the Gospel came to Israel and the covenant with Israel concerning the Gospel had already been made. The Law cannot possibly contribute to the salvation of those that God had already promised salvation apart from the Law.
A driving question to this day is why God sent the Law when men are not saved by it and in fact are condemned (at least the condemnation is manifested) by this Law. Galatians 3 gives us at the very least part of the answer. If the thought of why God sent the Law after He promised the Gospel has never been something you have thought of, then please give it some thought. Not only is this a revelation of the New Testament, it is something that is very relevant to our day. If God gave the Law with and for a purpose that the Law is not being used for, then it should make us nervous and wary about this. The Law was given so that all men would be shut up under sin.
This deserves to be repeated over and over until it is driven to the depths of our heads and our hearts. The Law was given so that all men would be shut up under sin. It was never given as something men could keep and thereby be holy, but instead it was given to show us our unholiness and inability to get better by the Law. It was given to show us our hearts and those hearts are provoked by the Law and thus we see that they are full of sin and are sinful by nature. The Law was so far from being given so that people could earn righteousness by it for salvation that the text tells us that it shut up everyone under sin so that the promise by faith in Christ might be given to those who believe. Notice the point and the glory of that point. The Law was not given in opposition to the Law, but it was given so that the promise could be carried out.
This point is so vital that it must be repeated because our hearts are drawn by a spiritual or fleshly gravity (so to speak) to the Law for blessings or for salvation. We live by the free-grace of God which is to say that we live by promise rather than by the Law. Before this faith came, which is to say, before Christ came who was the fulfillment of the promise and so people had faith in Him more specifically, we were in custody by the Law and we had no other place to go. The Law was used to shut people up under sin which shut people up to the faith later to be revealed. This shows the place of the Law and it was and is to lead people to Christ where the promises of the Gospel are fulfilled. The Law came 430 years after the promises of the Gospel and it can never supplant the Gospel or add to the Gospel. The Law leads and drives people to Christ and they are justified by faith alone.
We live in a day where free-grace is under attack from all sides. We live in a day where the true work of the Law has been changed from what it was supposed to be for to what it was not to be for and will never be for. The Law was to be a tutor or instructor of men and their hearts as to their sin and their sinful natures. The Law has no place in the Gospel itself and at best an advisory role in sanctification. Christ is the Gospel and Christ is our sanctification. The Law shows us our sin and is used by the Spirit to convict us of our sin and drive us out of self-love and self-strength into despair of self and looking to Christ alone. It must never have any other place.