15 Brothers and sisters, let me take an example from everyday life. Just as no one can set aside or add to a human covenant that has been duly established, so it is in this case. 16 The promises were spoken to Abraham and to his seed. Scripture does not say “and to seeds,” meaning many people, but “and to your seed,” meaning one person, who is Christ. 17 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. 18 For if the inheritance depends on the law, then it no longer depends on the promise; but God in his grace gave it to Abraham through a promise.
19 Why, then, was the law given at all? It was added because of transgressions until the Seed to whom the promise referred had come. The law was given through angels and entrusted to a mediator. 20 A mediator, however, implies more than one party; but God is one.
21 Is the law, therefore, opposed to the promises of God? Absolutely not! For if a law had been given that could impart life, then righteousness would certainly have come by the law. 22 But Scripture has locked up everything under the control of sin, so that what was promised, being given through faith in Jesus Christ, might be given to those who believe.
23 Before the coming of this faith, we were held in custody under the law, locked up until the faith that was to come would be revealed. 24 So the law was our guardian until Christ came that we might be justified by faith. 25 Now that this faith has come, we are no longer under a guardian.
26 So in Christ Jesus you are all children of God through faith, 27 for all of you who were baptized into Christ have clothed yourselves with Christ. 28 There is neither Jew nor Gentile, neither slave nor free, nor is there male and female, for you are all one in Christ Jesus. 29 If you belong to Christ, then you are Abraham’s seed, and heirs according to the promise. (NIV)
I won't typically take passages this long in our study of Galatians, but it's too easy to chop up this single argument into bits and then get it wrong because you don't see how the whole passage fits together and also because Paul's analogy in verse 15 can get bent severely out of shape if pressed too far and not understood in light of the following paragraphs. In this section of his argument Paul substantiates his earlier claim that the covenant people enter the family of God on the basis of faith by exploring the relationship between the Abrahamic covenant and the law. This is one of the more difficult and dense sections in the letter so there will probably be a couple of posts spun off from here dealing with specific issues. Here we will focus on his main points.
Paul begins in verse 15 by using an example from Greek and Roman law. Wills are unalterable by anyone other than the person who made the will. The same is true in this case. God made a promise to Abraham and to his seed that he would receive an inheritance from God of, among other things, a worldwide family. It's important that we notice here that Christ is the recipient of the promise. It is to him as Abraham's seed that a family is promised. The Gentiles join Abraham's family via Christ, the seed, not via the law which came 430 years later. The problem with the law is not just that it came later and did not alter the already given promise. It's also that the law and promise are antithetical just as works of the law and faith were.
This naturally raises the questions that Paul deals with next; why did God bother giving the law at all? Are verses 19 and 20 suggesting that God wasn't the origin of the law? While Paul is clearly distancing God from the law a bit in those verses, it isn't fair to suggest as some (e.g., Martyn) have that the law didn't have its genesis in God. Paul's distancing God from the law for the purpose of showing that it is of less value than the promise. The law came for a limited time (until Christ - the one to whom the promise was made - came) and a limited purpose. It was never intended to be the means by which one entered the people of God.
The purpose of the law was limited, but it wasn't in opposition to the promise. The law has a 'negative' role, dealing with sin. More specifically the law was given to Israel, cursing them by displaying their inability to live God's way apart from the Spirit. They failed just as Adam failed. By extension it shows the inability of all to live God's way. Additionally, the law didn't free Israel from the power of sin, even though the sacrificial system was in place and did deal with sin in some limited sense. They, like all the rest were still under sin's power. The law also had a positive role that was related to its negative role, and this is where Israel was at an advantage. It was a means to protect Israel from the power of sin until Christ could come. Here it's worth noting that Paul isn't giving a complete theology of the law. Rather he's dealing with a very specific question: what is the role of the law in becoming a member of the people of God (none) and how did the law relate to the covenant (it's subordinate to it and preserved Israel so that the seed could come and show us the way to become true children of Abraham - by faith and to enable that path through his faithfulness). Additionally this will pave the way for Paul's later discussion of the Spirit as that which guides God's people.
Verses 26-29 sum up the argument thus far and also extend it with an argument from the Christian tradition. In verse 26 Paul affirms that Jew and Gentile alike are part of Abraham's family on the basis of faith and faith alone. Through baptism the Galatians came to be completely identified with Christ on the basis of their union with him. "For Paul, it is the participationist soteriology of being 'in Christ' that bridges the expanse between Abraham and the Gentile world, and not Torah observance as the Judaizers argued" (Longenecker 151). The Galatians were united with Christ and clothed with him, meaning that part of their identification with Christ involved a moral transformation - the way they lived looked like Christ, they had entered into the new creation, the new era, and thus, again, the law was unnecessary. He further substantiates that point by citing what in all likelihood was a snippet from an early Christian baptismal liturgy, possibly the one he himself used when baptizing his converts. The Jew/Greek distinction is irrelevant in Paul's eyes. At baptism both have entered the people of God by faith and both are full covenant members. There's no need for Gentiles to follow the law.