John Calvin Commentary Galatians 2:16

"yet knowing that a man is not justified by the works of the law but through faith in Jesus Christ, even we believed on Christ Jesus, that we might be justified by faith in Christ, and not by the works of the law: because by the works of the law shall no flesh be justified." — Galatians 2:16 (ASV)

But by the faith of Jesus Christ. He does not merely state that ceremonies, or works of any kind, are insufficient without the assistance of faith, but counters their denial with a statement that allows no exception, as if he had said, “Not by works, but by the Gift of Christ alone.” From any other viewpoint, the sentiment would have been trivial and irrelevant; for the false apostles did not reject Christ or faith, but demanded that ceremonies should be joined with them. If Paul had admitted this claim, they would have been in perfect agreement, and he would not have needed to agitate the church with this unpleasant debate.

Therefore, let it remain settled that the proposition is so framed as to allow no exception: “that we are justified in no other way than by faith,” or, “that we are not justified except by faith,” or, which amounts to the same thing, “that we are justified by faith alone.”

Thus, it becomes clear with what foolish quibbling the Papists of our day dispute with us about the word, as if it were a word we invented. But Paul was unfamiliar with the theology of the Papists, who declare that a person is justified by faith, and yet make a part of justification consist in works. Of such half-justification Paul knew nothing.

For, when he instructs us that we are justified by faith because we cannot be justified by works, he assumes as true that we cannot be justified through the righteousness of Christ unless we are lacking and devoid of our own righteousness. Consequently, either nothing or everything must be ascribed to faith or to works.

As for the word justification, and the manner in which faith is its cause, we will see later.

By the works of the law shall no flesh be justified. He had already appealed to the consciences of Peter and others, and now confirms it more fully by affirming that it is indeed true: by the works of the law no mortal will obtain justification.

This is the foundation of a freely given righteousness, when we are stripped of our own righteousness. Furthermore, when he asserts that no mortal is justified by the righteousness of the law, the assertion means that all mortals are excluded from this way of justification, and that no one can possibly attain it.