Thomas Aquinas — on Gal 2:21 (Commentary on Galatians)

“Then when he says, "I cast not away the grace of God," he draws the principal conclusion. First, he draws the conclusion; secondly, he explains it. He says, therefore: Because I have received from God so great a grace that He delivered Himself, and I live in the faith of the Son of God, "I cast not away the grace of God," i.e., I do not repudiate it or show myself ungrateful: "The grace of God in me hath not been void, but I have labored more abundantly than all they" (1 Cor 15:10). Hence another version has, "I am not ungrateful for the grace of God." "Looking diligently lest any man be wanting to the grace of God" (Heb 12:15), i.e., by showing myself unworthy because of ingratitude. A form of repudiation and of ingratitude would exist, if I were to say that the Law is necessary in order to be justified. Hence he says, "For if justice be by the law, then Christ died in vain," i.e., if the Law is sufficient, i.e., if the works of the Law suffice to justify a man, Christ died to no purpose and in vain, because He died in order to make us just: "Christ also died once for our sins, the just for the unjust, that he might offer us to God" (1 Pet. 3:18). Now if this could have been done through the Law, the death of Christ would have been superfluous. But He did not die in vain or labor to no purpose, as it is said in Isaiah (49:4); because through Him alone came justifying grace and truth, as it is said in John (1:17). Therefore, if any were just before the passion of Christ, this too was through the faith of Christ to come, in Whom they believed and in Whose faith they were saved.”