Here, the subject is the role faith and works play in the justification process. This time, he uses Abraham as the model by which all his "children in the faith" also become "children of God." He begins by posing a question, which can be paraphrased as, "Do miracles come by ritual?" There is in this a veiled illusion to magic. Do miracles come by incantation? Do they come by knowing certain formulas that may include even such things as cutting the flesh or going through long periods of fasting or sufferings to get God's attention? Will God respond with a miracle out of pity once we show Him how humble and righteous we are? No, it does not work that way. Miracles come by a living God, who is actively working in our lives because He called us and we have faith in Him.
With that foundation, Paul begins what turns into the preamble for a very controversial section of Galatians. He proceeds to state that it was through faith that Abraham was justified. It is good to remember that Abraham not only believed who God is, but he also believed what God said. This is what set him apart from everybody else. His faith was not merely an intellectual agreement, but he also lived His faith.
Abraham's works did not win him acceptance by God, but they did prove to God that Abraham really did believe Him. So Paul says in verses 10-11 that those who rely on their works to justify them are under the curse of the law. What is "the curse of the law"? The death penalty! When one sins, he brings on himself the curse of the law he broke, which is death. In effect, he says that those who seek justifcation through works are still under the curse because justification by this means is impossible.
So powerful is the curse of the law that, when our sins were laid on the sinless Jesus Christ, the law claimed its due. Jesus died! Paul quotes from Deuteronomy 27:26 to counteract those who were troubling the church, because they were saying that their asceticism, magic, and similar things (like keeping Halakah, the oral law and traditions of Judaism) could justify.