John Calvin Commentary Romans 2:14

"(for when Gentiles that have not the law do by nature the things of the law, these, not having the law, are the law unto themselves;" — Romans 2:14 (ASV)

For when the Gentiles, etc. He now states what proves the former clause; for he did not think it enough to condemn us by mere assertion, and only to pronounce on us the just judgment of God, but he proceeds to prove this by reasons, in order to excite us to a greater desire for Christ, and to a greater love toward him.

He indeed shows that ignorance is futilely claimed as an excuse by the Gentiles, since they prove by their own deeds that they have some rule of righteousness: for there is no nation so lost to everything human that it does not keep within the limits of some laws. Since, then, all nations, on their own and without an instructor, are disposed to make laws for themselves, it is evident beyond all question that they have some notions of justice and rectitude, which the Greeks call preconceptions (προληψεις), and which are implanted by nature in human hearts.

They have, then, a law, though they are without the Law: for although they do not have a written law, they are nevertheless by no means entirely destitute of the knowledge of what is right and just, as they could not otherwise distinguish between vice and virtue; the first of which they restrain by punishment, and the latter they commend, showing their approval of it by honoring it with rewards. He sets nature in opposition to a written law, meaning that the Gentiles had the natural light of righteousness, which took the place of that law by which the Jews were instructed, so that they were a law to themselves.