Continuing in Galatians Chapter 3, Paul was providing these believing Gentiles with a hard-hitting explanation of why they do not want to pursue works of the law—which they were never under—in order to perfect themselves. They began in the Spirit and were being coerced into thinking they needed Jesus plus works in order to be justified. This was a common mindset among believing Jews in the early church. After all, it wouldn't be "fair" for believing Jews to be required to continue in the law while non-Jewish people were let off the hook regarding that burdensome ministry.
Redemption for both Jews and Gentiles came by a promise, through faith ... and the law was not of faith. It came by inheritance, gifted to us who have received the promise of the Spirit through faith.
Paul appears frustrated as he addresses "you foolish Galatians" in the beginning of Chapter 3. Why? They had started out receiving the Spirit of God by faith but were being lured by a more popular persuasion from men who came from James—and had started to convince these Gentiles to adhere to the old Jewish law in combination with faith in Christ. This led to Paul carefully laying out an explanation over the next couple of chapters or so of how works and faith could not be mixed when it came to their salvation and right standing with God.
Abraham and the foundational message of a promise made to him by God will be Paul's launching pad to explain how the law was a curse and was not of faith. It will be fruitless and ineffective for them to think perfection could come through works (the flesh).