“When John had first preached before his coming the baptism of repentance to all the people of Israel.”
King James Version (KJV)
13:24 John having first preached - He mentions this, as a thing already known to them. And so doubtless it was. For it gave so loud an alarm to the whole Jewish nation, as could not but be heard of in foreign countries, at least as remote as Pisidia.
Ac 13:24,25 John had first preached. John, who preached before the Savior's coming, is named because he was well known to the Jews, and most of them regarded him a prophet. See Mt 3:1-12 Joh 5:32-35.