Meanwhile, Paul, while proclaiming our freedom from the law, shows how the law works in the purposes of God. (Gal 3:10-29) It is true that the law is inferior to the promise made to Abraham, but it is also true that the law is given by God for a specific purpose. So Paul does not proclaim us free from the divinely ordained purpose of the law, (Gal 3:21) but rather from the Pharisaical way that the law had been taught by both the Pharisees and the Judaizers.
The Pharisees and Judaizers both looked to their own observance of the law for salvation, but they especially looked to their circumcision, which was a sign for them that a person was a law-keeper. It is for this reason that Paul tells the Galatians "...if you accept circumcision, Christ will be of no advantage to you." (Gal 5:2) The message that Paul has for the Galatians is that they should not look to their observance of the law for salvation, but to look to the One who actually fulfilled the law for salvation. (Gal 3:13 and 4:4-5) So in this most basic sense the book is about justification.
On the other hand, the Judaizers were teaching that to grow in the Christian faith, you must learn to keep the Mosaic law (the Ten Commandments, the Ceremonies, the dietary laws, the festivals, etc). Paul responds to this error by reminding the Galatians of the means of their salvation, faith alone. (Gal 3:1-7) They had been justified by faith without the law, they had received the Spirit by faith without the law, and they had experienced miracles by faith without the law, therefore, Paul argues, "...Having begun by the Spirit, are you now being perfected by the flesh?" (Gal 3:3) So the book is also about sanctification by faith.
The Galatians are never told by Paul to keep the law in order to be sanctified. Instead he tells them that whoever turns back to the law is turning back to slavery and bondage. So rather than being a means of spiritual maturity, the law is a means of spiritual 'dwarfing'.
In the same way we also, when we were children, were enslaved to the elementary principles of the world. (Gal 4:3)Further, Paul says that those who have faith are the mature heirs of God and the promises of Abraham, and that those who are under the law are still children who have not grown into maturity. (Gal 3:23-4:7)
But now that you have come to know God, or rather to be known by God, how can you turn back again to the weak and worthless elementary principles of the world, whose slaves you want to be once more? (Gal 4:9)
Then, in the practical section of the book (chapters 5 and 6), Paul tells the Galatians to hold fast to their freedom that they have been given by Christ. This freedom is not to be abused through indulging the flesh but, "...through love serve one another," (Gal 5:13) since, "...the whole law is fulfilled in one word: 'You shall love your neighbor as yourself.'" (Gal 5:14)
Paul goes on to explain that whoever is led by the Spirit is not under the law (Gal 5:18) because, "...the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control; against such things there is no law." (Gal 5:22-23)
Notice the phrase, "against such things there is no law". Paul here is saying that the Spirit will not lead us to do things contrary to the law of God, rather through the Spirit which we have received by faith, we fulfill the law, because the Spirit leads us to exhibit this fruit towards our neighbors.
So sanctification for Paul is not accomplished by a list of do's and don'ts or observance of the law, but by the Spirit of God who lives in us makes us new creatures;
For neither circumcision counts for anything, nor uncircumcision, but a new creation. (Gal 6:15)Finally, Paul reminds the Galatians of the marks that really count, not the marks of circumcision, but he says, "From now on let no one cause me trouble, for I bear on my body the marks of Jesus." (Gal 6:17)