Adam Clarke on Acts 9:20

Preached Christ in the synagogues - Instead of ΧριϚον, Christ, Ιησουν, Jesus, is the reading of ABCE, several others of high importance, together with the Syriac, Coptic, Ethiopic, Armenian, Slavonic, and Vulgate.

The great question to be determined, for the conviction of the Jews, was that Jesus was the Son of God. That the Christ, or Messiah, was to be the Son of God, they all believed. Saul was now convinced that Jesus, whom they had crucified, and who had appeared to him on the way, was the Son of God, or Messiah; and therefore as such he proclaimed him. The word Christ should be changed for Jesus, as the latter is, without doubt, the genuine reading.

The first offers of the grace of the Gospel were uniformly made to the Jews. Saul did not at first offer Jesus to the heathens at Damascus; but to the synagogues of the Jews.