14 Tell me certainly, that you may be confounded by that lamp by which also were the former enemies confounded, who were like to you, the Pharisees, who, when they questioned the Lord by what authority He did those things: “I also,” said He, “will ask you this question, Tell me, the baptism of John, whence is it? From heaven, or of men?” And they, who were preparing to spread their wiles, were entangled by the question, and began to debate with themselves, and say, “If we shall answer, It is from heaven, He will say unto us, Wherefore did ye not believe him?”
For John had said of the Lord, “Behold the Lamb of God, who takes away the sin of the world!” Why then do you inquire by what authority I act? O wolves, what I do, I do by the authority of the Lamb. But that you may know the Lamb, why do you not believe John, who said, “Behold the Lamb of God, who takes away the sin of the world”? They, then, knowing what John had said regarding the Lord, said among themselves, “If we shall say that John's baptism is from heaven, He will say unto us, Wherefore then did ye not believe him?
If we shall say, It is of men, the people will stone us; for they hold John as a prophet.” Hence, they feared men; hence, they were confounded to confess the truth. Darkness replied with darkness; but they were overcome by the light. For what did they reply? “We know not;” regarding that which they knew, they said, “We know not.” And the Lord said, “Neither tell I you by what authority I do these things.” And the first enemies were confounded. How? By the lamp. Who was the lamp? John. Can we prove that he was the lamp? We can prove it; for the Lord says: “He was a burning and a shining lamp.” Can we prove also that the enemies were confounded by him? Listen to the psalm: “I have prepared,” he says, “a lamp for my Christ. His enemies I will clothe with shame.”
Source: Tractates on the Gospel of John (New Advent)