9 Let us see, then, what answer John gives: “They came unto John, and said unto him, Rabbi, he that was with you beyond Jordan, to whom you bore witness, behold the same baptizes, and all men come to him:” that is, What do you say? Ought they not to be hindered, that they may rather come to you? “He answered and said, A man cannot receive anything, unless it be given him from heaven.” Of whom, think you, had John said this? Of himself. “As a man, I received,” says he, “from heaven.”
Note, my beloved: “A man cannot receive anything, unless it be given him from heaven. You yourselves bear me witness that I said, I am not the Christ.” As much as to say, “Why do ye deceive yourselves? See how you have put this question before me. What have you said to me? 'Rabbi, he that was with you beyond Jordan, to whom you bore witness.' Then you know what sort of witness I bare to Him. Am I now to say that He is not the same whom I declared Him to be? And because I received somewhat from heaven, in order to be something, do you wish me to be empty of it, so as to speak against the truth? 'A man cannot receive anything, unless it be given him from heaven.
You yourselves bear me witness that I said I am not the Christ.'” You are not the Christ; but what if you are greater than He since you baptized Him? “I am sent:” I am the herald, He is the Judge.
Source: Tractates on the Gospel of John (New Advent)