Albert Barnes Commentary Acts 9:20

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

And straightway. Immediately. This was evidence of the genuineness of his conversion, as he was willing to immediately avow himself as the friend of the Lord Jesus.

He preached Christ. He proclaimed and proved that Jesus was the Christ .

Many manuscripts read Jesus here instead of Christ. Griesbach adopted this reading. It is also the reading of the Syriac, Vulgate, and Ethiopic versions. This reading aligns much better with the subject matter than the common reading.

All admitted that Christ, or the Messiah, was the Son of God. In the New Testament, the names Christ and Son of God are used synonymously. However, the question was whether Jesus was the Christ, or the Son of God, and Paul showed this to the Jews.

Paul continued the practice of attending synagogues. In these, anyone had a right to speak if invited by the officiating minister (Acts 13:15).

That he is the Son of God. That he is the Messiah.