Martin Luther on Galatians 2:5
Since our opponents will not let it stand that only faith in Christ justifies, we will not yield to them. On the question of justification we must remain adamant, or else we shall lose the truth of the Gospel. It is a matter of life and death. It involves the death of the Son of God, who died for the sins of the world. If we surrender faith in Christ, as the only thing that can justify us, the death and resurrection of Jesus are without meaning; that Christ is the Savior of the world would be a myth. God would be a liar, because He would not have fulfilled His promises. Our stubbornness is right, because we want to preserve the liberty which we have in Christ. Only by preserving our liberty shall we be able to retain the truth of the Gospel inviolate.
Commentary on the Epistle to the Galatians (1535)