Paul's letter to the Galatians could be called the Christian Declaration of Independence, as Cliffs Notes points out. It's all about Paul telling the Galatians (in an area north of modern Turkey) they shouldn't listen to the Jewish infiltrating notion of living up to the law only, that it's more important to live up to the spirit of Christianity. Loving each other is more important that just focusing on the rules. The spirit of the law, rather than the letter of law.
Galatians could also be called the Book of Circumcision. Paul goes on at length about how God doesn't judge a person by his/her external self, but by his/her spirit, or character, and Christians therefore shouldn't worry about getting circumcised.