Albert Barnes Commentary John 3:28

"Ye yourselves bear me witness, that I said, I am not the Christ, but, that I am sent before him." — John 3:28 (ASV)

Bear me witness. You remember that at first I told you I was not the Messiah. As he had been witness to Jesus—since he came for no other purpose than to point him out to the Jews—they should not suppose that he was his superior. It was only reasonable to expect that Christ himself would be more successful than his forerunner.

"I came, not to form a separate party or a distinct sect, but to prepare the way so that he might be more successful, that the people might be ready for his coming, and that he might achieve the success he has actually experienced. You should rejoice, therefore, at that success and not be troubled by it, for his success is the best proof of the greatness of my message, and of its success also."