John Calvin Commentary Acts 9:20

"And straightway in the synagogues he proclaimed Jesus, that he is the Son of God." — Acts 9:20 (ASV)

Luke now declares how fruitful Paul’s conversion was, namely, that he went out immediately, and not only professed that he was a disciple of Christ, but also set himself against the fury and hatred of the enemies by resolutely defending the gospel.

Therefore, he who recently ran headlong against Christ with furious force, now not only meekly submits himself to His will, but like a resolute standard-bearer, fights even in the face of extreme danger to maintain His glory. It is certain that he was not so quickly shaped by Ananias’ efforts, but as soon as he had learned the first principles from a human source, he was afterward raised by God to higher things.

He briefly summarizes the essence of his preaching when he says that Christ was the Son of God. In the same sense, he says shortly after that he saw Christ. And understand this: when Paul preached from the Law and the Prophets about the true office of the Messiah, he also taught that everything that was promised concerning, and was to be hoped for from, the Messiah was revealed and given in Christ.

For the words mean this much when he says that he preached that Christ is the Son of God. That was undoubtedly a principle among the Jews: that a Redeemer would come from God, who would restore all things to a blessed state. Paul teaches that Jesus of Nazareth is this Redeemer, which Paul cannot do unless he shakes off those gross errors he had conceived about the earthly kingdom of the Messiah.

It is certain that Paul declared how Christ was promised in the Law, and for what purpose. But because all this aimed at proving that the Son of Mary was the One to whom the Law and the Prophets bore witness, Luke is therefore content with this single point only.