I do not frustrate the grace of God: for if righteousness [come] by the law, then Christ is dead in vain.
Note 16 at Ga 2:21: Paul's statement implies that the grace of God can be frustrated. The word "frustrate" means "to prevent from accomplishing a purpose or fulfilling a desire; thwart" (American Heritage Dictionary). The purpose of God's grace in our lives is to set us free from every device of Satan (1Jo 3:8) and grant us abundant life (see note 6 at Joh 10:10). This will not come to pass if we are striving to approach unto God through keeping the Law.
Note 17 at Ga 2:21: What a statement! This should leave no doubt as to whether we can be justified through the Law. "For Christ is the end of the law for righteousness" (Ro 10:4), so anyone who is trusting in the Law is not trusting in Christ.