Adam Clarke on Acts 26:20

But showed first unto them of Damascus - He appears to have preached at Damascus, and in the neighboring parts of Arabia Deserta, for about three years; and afterwards he went up to Jerusalem. See Gal 1:17, Gal 1:18; and see the note on Act 9:23.

That they should repent - Be deeply humbled for their past iniquities, and turn to God as their Judge and Savior, avoiding all idolatry and all sin; and thus do works meet for repentance; that is, show by their conduct that they had contrite hearts, and that they sincerely sought salvation from God alone. For the meaning of the word repentance, see the note on Mat 3:2.