"What I mean is this: The law, introduced 430 years later, does not set aside the covenant previously established by God and thus do away with the promise" (Galatians 3:17)
Paul’s sixth argument is that the law as a covenant was not designed to supplant grace but to show the need for grace (3:15–5:12). The law was not an alternative way of salvation and was never given for that reason. Rather, the law was designed to show the way for redeemed people to please God. Moreover, because the law exposes our sins, it continually drives us back to faith as the only way to achieve life.
Paul has established that Abraham and his children, including the Gentiles, are saved by faith in God’s promise. What, then, was the purpose of the law? The law was given 430 years after Abraham entered the Promised Land, and thus was added to the promise. First came promise, then law. The law was not designed to replace the promise. After all, the Israelites continued to be the children of Abraham and continued to keep the Abrahamic covenant by believing the promises.
What then was the law’s purpose? Paul says that the law was added “because of transgressions” (Galatians 3:19). It was designed to show us our sins and point out our need for a savior. Considered as a covenant, the law was designed as a prison, to keep the people locked up so that their sins would not get out of hand (3:22–23). Before the Flood, sin progressed so far that God made the decision to destroy the world. To prevent this from happening again, God instituted the law and its penalties as a way of holding back the progress of sin. The law, however, could not take away sin and give salvation. Thus, the law was only temporary, and pointed forward to the new covenant, when sin would be restrained, the penalty paid, the Spirit poured out, and the progress of evil reversed.
Paul said now that the new covenant has come, we are no longer under the law’s supervision. Its function is completed. We are now sons of God, clothed in Christ, and empowered to live righteously (3:24–27). The promise first made to Abraham had come to pass, and the period of the law’s supervision, joyously, had come to an end.
The proper function of the law is to put before us the right way of living before God. This is very nearly the definition of what it means to lived under God's authority and unto His glory. Make good use of God’s law as you live your life for God. Pursue the law for Christian growth, and in so doing, please God.