Paul reasons with the Galatians that the law did not cancel the promise to Abraham, nor was it God's plan for saving Israel. This brings up the natural question, "Wait a minute. Why was the law given to Israel?" So beginning in verse 19 of chapter 3 Paul begins to deal with this.
First, we need to see the promise to Abraham as two bookends. God had given Abraham his Promise, which he was to hold onto by faith, until his Seed would come who would bless all the earth. Paul inserts in verse 16 that the Seed God was pointing to was Jesus, the messiah. Thus there is this period of nearly 2,000 years that the descendants of Abraham needed to hold onto this promise by keeping their faith in God. However, God knew, and even hinted to Abraham, that his descendants would have trouble. When we think of the context of Moses leading Israel out of Egypt and giving them the law, we see that they were in bondage physically. But, worse they were in bondage spiritually. They had begun to worship the gods of the Egyptians and yet, they still had 1400 years until the messiah. They weren't going to make it and I am not just talking spiritually. This leads us to the first purpose of the law, which is to restrain the outward actions and effects of sin. This added restraint would serve to help Israel survive as a nation until Jesus came. As it is, even with the law, God removed them from the land for 70 years and then brought them back.
Notice that Paul says that the law was added until Messiah. That means Jesus is the end of God's desire to use the Law within Israel to restrain the growth of sin. Of course the people of Israel saw the Law as their means of salvation. So all this is quite puzzling to the religious leaders of Jesus day. In fact, it was more than puzzling, it was heresy to them.
The next question Paul answers is whether or not the Law was "against" the promises of Abraham. The idea here is that if the law doesn't get rid of the promise then does it somehow conflict or interfere with the promises. In fact, without getting too technical, the word translated against has more the sense of this, is the Law better or above the promises. Paul's answer to that is a very strong negation, certainly not! If the law could have given anyone life then God would have used the righteousness of the Law to justify people. Only the Promise can give life. This leads to Paul's second reason for the law.
Not only did sin need restrained, but Israel needed to be trained so that they would be able to recognize the messiah and understand what he was doing. In fact the metaphor given by Paul is that Israel under the law was like a young child being trained by a tutor to prepare them for life as an adult. In fact, we can see this analogy in a simpler form, which is pointed out in Hebrews 12:6-11. That form is the family. Parents train their children, as they see fit, in order to prepare them for life. Many "laws" that parents give are not purely moral issues. They are created to prepare the kid for doing well in life. Here is an example. A parent may tell a child that they have to go to bed by a certain time at night. We can debate about whether that is a good rule or not. But no one can really argue that there is something inherently good about the hour of 9:00 PM that causes all who go to sleep at that time to be blessed somehow. In other words, there is nothing inherently evil in a child going to bed at 9:30 PM. So why does a parent make such a law? For various reasons. Some that have immediate applications, such as, restraining a child's immature desire to never go to bed. However, there are also future implications, such as, knowing how much discipline is needed in going to bed to be able to faithfully satisfy the requirements of a job or business, and taking care of a family. These type of "laws" are like training wheels that are meant to come off when the child is able to keep their balance on their own. Granted, the Law of Moses did have some purely moral commands, you shall not kill. Killing will always be an obvious moral wrong. But the Law also had preparatory laws that were meant to be "dropped" off when Israel reached national adulthood, which was initiated by the coming of Jesus the Messiah.
The Law restrained sin, and prepared Israel to recognize messiah, but there was one more thing it did. This is shown in verse 22. For the person who truly loved God and worked hard to fulfill all the requirements of the law there would be a dark side to the law. Or better yet, it would reveal a dark side within them. No one is perfect and even when we are successful on the public persona our private self feels the condemnation of the law. Thus the Law ends up imprisoning each and every son or daughter of the law under the condemnation and guilt of God's wrath. This may be a horrible idea, but the point is that those who were being honest with themselves would be driven to the place David was in Psalm 51. Which is quite simply a series of recognitions:
The Pharisees' problem was that they hypocritically justified their sin and "muted" the Laws ability to teach them their need for Jesus. When he tried to show them this they hated him and "muted" him by crucifying him on a cross. The law is a witness to us that we are sinners and need God to come down and do for us what we cannot do for ourselves. It is the foundation to the gospel that allows us to stare at the cross and go from horror to brokenness to surrender, and lastly to joy.
Now here is a very important point. In verse 25 Paul says that Faith takes the place of the tutor or the Law. We have a tendency to think that grace got rid of the law. Actually grace satisfied the law and made Jesus the known object of our faith, rather than a "hoped for" act of God. Let me just say this. Paul makes it clear that no one has ever been saved by the Law, they have only been cursed by it. However, in that curse is a blessing for those who will humble themselves and simply wait upon God. How can they do that? By faith. Abraham was saved by faith that looked forward to an unknown blessing of God. David was saved by believing that God would make a way for his sins to be covered. Isaiah was saved by trusting God would do what he said he would do, make salvation by his own right arm." Isaiah waited on the Lord in faith, even though it didn't come in his life time.
Paul goes on to talk about the Galatians current "adult" status in God. Because of their faith in Jesus they were now sons of God who were no longer under the Law. In fact he points out that those who are baptized in Jesus' name are actually baptized into him and, also put him on. This may seem puzzling but remember that baptism symbolized death. Just as Jesus died and was resurrected with the new glorified body provided, so we too die to our old self and take on a new spiritual body. Definitely these things will be literal and actual in the Ressurrection. However they have a spiritual application in the now. Christ becomes the body that we cloth ourselves with spiritually. I no longer live to please my dead, rotting, flesh but live in order to demonstrate the life and mind of Christ by the help of his indwelling Holy Spirit.
Paul points out in verse 28 that the old distinctions of the Law no longer apply. Why? Because both Jews and Gentiles are in the same boat, sinners under God's condemnation. All may be a part of God's family by faith in the only one who ever "made it" on his own merits, Jesus. What were those old distinctions that no longer have spiritual implications? Race. Economic social status. Gender. We all become Abraham's seed in Jesus. In fact Jesus is the true "melting pot" of the world. In this sense America doesn't even come close.