John Gill 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)

You yourselves bear me witness
In what they now said, and referred to, in describing Christ, as he to whom John bore witness; and he appeals to them for what he said in their presence, and before all the people in that testimony: that I said I am not the Christ ;
see (John 1:19John 1:20) ;

wherefore, if he, who is the Christ, is now come, and teaches, and baptizes, and has the greatest number of followers, it is not to be wondered at; and much less to be envied; but rather to be rejoiced at.

For John, by repeating what he had before said, that he was not the Christ, suggests, that Jesus was; and therefore was superior to him in office, honour, and usefulness, who was only his harbinger and forerunner, as follows: but I am sent before him ;

to prepare his way, to make straight his paths, to proclaim his coming, and kingdom, being at hand; and that the expectations of men may be raised, and they may be ready to receive him when come: and hereby the end of John's office, and ministry, had its accomplishment.

The Ethiopic version adds, "to preach him".