...know that a man is not justified by observing the law, but by faith in Jesus Christ. So we, too, have put our faith in Christ Jesus that we may be justified by faith in Christ and not by observing the law, because by observing the law no one will be justified.This verse was used effectively against those in the Catholic Church who preached that only by doing good deeds could a person get into heaven. So much did Martin Luther love this book that he once said "The Epistle to the Galatians is my epistle. To it I am as it were in wedlock. It is my Katherine." The Katherine he refers to is his wife.
Galatians 2:16
If you are interested in learning more click here to read some of Luther's commentary on Galatians.