3. He preached Christ in the synagogues, v. 20. To this he had an extraordinary call, and for it an extraordinary qualification, God having immediately revealed his Son to him and in him, that he might preach him, Gal 1 15, 16. He was so full of Christ himself, that the Spirit within him constrained him to preach him to others, and, like Elihu, to speak that he might be refreshed, Job 32 20. Observe, (1.) Where he preached—in the synagogues of the Jews, for they were to have the first offer made them. The synagogues were their places of concourse; there he met with them together, and there they used to preach against Christ and to punish his disciples, by the same token that Paul himself had punished them oft in every synagogue (ch. 26 11), and therefore there he would face the enemies of Christ where they were most daring, and openly profess Christianity where he had most opposed it. (2.) What he preached: He preached Christ. When he began to be a preacher, he fixed this for his principle, which he stuck to ever after: We preach not ourselves, but Christ Jesus our Lord; nothing but Christ, and him crucified. He preached concerning Christ, that he is the Son of God, his beloved Son, in whom he is well pleased, and with us in him, and not otherwise. (3.) How people were affected with it (v. 21): All that heard him were amazed, and said, "Is not this he that destroyed those who called on this name in Jerusalem, and now does he call on this name himself, and persuade others to call upon it, and strengthen the hands of those that do?" Quantum mutatus ab illo—Oh how changed! Is Saul also among the prophets? Nay, did he not come hither for that intent, to seize all the Christians he could find, and bring them bound to the chief priests? Yes, he did. Who would have thought then that he would ever preach Christ as he does? Doubtless this was looked upon by many as a great confirmation of the truth of Christianity, that one who had been such a notorious persecutor of it came, on a sudden, to be such an intelligent, strenuous, and capacious preacher of it. This miracle upon the mind of such a man outshone the miracles upon men's bodies; and giving a man such another heart was more than giving men to speak with other tongues.

Matthew Henry Commentary on the Whole Bible (Concise) (Matthew Henry)

Verses 10-22 A good work was begun in Saul, when he was brought to Christ's feet with those words, Lord, what wilt thou have me to do? And never did Christ leave any who were brought to that. Behold, the proud Pharisee, the unmerciful oppressor, the daring blasphemer, prayeth! And thus it is even now, and with the proud infidel, or the abandoned sinner. What happy tidings are these to all who understand the nature and power of prayer, of such prayer as the humbled sinner presents for the blessings of free salvation! Now he began to pray after another manner than he had done; before, he said his prayers, now, he prayed them. Regenerating grace sets people on praying; you may as well find a living man without breath, as a living Christian without prayer. Yet even eminent disciples, like Ananias, sometimes stagger at the commands of the Lord. But it is the Lord's glory to surpass our scanty expectations, and show that those are vessels of his mercy whom we are apt to consider as objects of his vengeance. The teaching of the Holy Spirit takes away the scales of ignorance and pride from the understanding; then the sinner becomes a new creature, and endeavours to recommend the anointed Saviour, the Son of God, to his former companions.