Albert Barnes Commentary Acts 13:25

"And as John was fulfilling his course, he said, What suppose ye that I am? I am not [he]. But behold, there cometh one after me the shoes of whose feet I am not worthy to unloose." — Acts 13:25 (ASV)

And as John fulfilled his course. As he was engaged in completing his work. His ministry is called a course or race; that which was to be run or completed.

He said, etc. These are not the precise words which the evangelists have recorded, but the sense is the same. See Barnes on John 1:20, and see Barnes on Matthew 3:11.