3 Yet because He appeared as it were in the night, in a mortal body, He lighted for Himself a lamp by which He might be seen. That lamp was John, concerning whom you lately heard many things: and the present passage of the evangelist contains the words of John; in the first place, and it is the chief point, his confession that he was not the Christ. But so great was the excellence of John, that men might have believed him to be the Christ: and in this he gave a proof of his humility, that he said he was not when he might have been believed to have been the Christ; therefore, “This is the testimony of John, when the Jews sent priests and Levites to him from Jerusalem to ask him, Who are you?” But they would not have sent unless they had been moved by the excellence of his authority who ventured to baptize. “And he confessed, and denied not.” What did he confess? “And he confessed, I am not the Christ.”
Source: Tractates on the Gospel of John (New Advent)