John Calvin Commentary Romans 1:2

"which he promised afore through his prophets in the holy scriptures," — Romans 1:2 (ASV)

Which he had before promised, etc. — Since the suspicion of being new greatly diminishes the authority of a doctrine, he confirms the faith of the gospel by its antiquity. It is as if he were saying: “Christ did not come to the earth unexpectedly, nor did He introduce a doctrine of a new kind, unheard of before, because He, and His gospel too, had been promised and expected from the beginning of the world.”

But since antiquity is often legendary, he brings witnesses, and approved ones, even the Prophets of God, so that he might remove every suspicion. He then adds, in the third place, that their testimonies were properly recorded, that is, in the Holy Scriptures.

We may learn from this passage what the gospel is: he teaches us that it was not proclaimed by the Prophets but only promised. If, then, the Prophets promised the gospel, it follows that it was revealed when our Lord was finally manifested in the flesh. Therefore, those who confound the promises with the gospel are mistaken, since the gospel is properly the appointed preaching of Christ as manifested, in whom the promises themselves are exhibited.