2 When our Lord Jesus Christ had spoken these things, the Jews answered, “You bear record of yourself; your record is not true.” Before our Lord Jesus Christ came, He lighted and sent many prophetic lamps before Him. Of these was also John Baptist, to whom the great Light itself, which is the Lord Christ, gave a testimony such as was given to no other man; for He said, “Among them that are born of women, there has not risen a greater than John the Baptist.” Yet this man, than whom none was greater among those born of women, said of the Lord Jesus Christ, “I indeed baptize you in water; but He that is coming is mightier than I, whose shoe I am not worthy to loose.” See how the lamps submits itself to the Day.
The Lord Himself bears witness that the same John was indeed a lamp: “He was,” says He, “a burning and a shining lamp; and you were willing for a season to rejoice in his light.” But when the Jews said to the Lord, “Tell us by what authority you do these things,” He, knowing that they regarded John the Baptist as a great one, and that the same whom they regarded as a great one had borne witness to them concerning the Lord, answered them, “I also will ask you one thing; tell me, the baptism of John, whence is it?
From heaven, or from men?” Thrown into confusion, they considered among themselves that, if they said, “From men,” they might be stoned by the people, who believed John to be a prophet; if they said, “From heaven,” He might answer them, “He whom you confess to have been a prophet from heaven bore testimony to me, and you have heard from him by what authority I do these things.” They saw, then, that whichever of these two answers they made, they would fall into the snare, and they said, “We do not know.”
And the Lord answered them, “Neither tell I you by what authority I do these things.” “I tell you not what I know, because you will not confess what you know.” Most justly, certainly, were they repulsed, and they departed in confusion; and that was fulfilled which God the Father says by the prophet in the psalm, “I have prepared a lamp for my Christ” (the lamp was John); “His enemies I will clothe with confusion.”
Source: Tractates on the Gospel of John (New Advent)