9 Therefore He showed that for the sake of men He desired to have Himself revealed by a lamp to the faith of those who believed, that by means of the same lamp His enemies might be confounded. There were enemies who tempted Him, and said, “Tell us by what authority do you do these things?” “I also,” says He, “will ask you one question; answer me. The baptism of John, whence was it? From heaven, or of men? And they were troubled, and said among themselves, If we shall say, From heaven, he will say unto us, Why did ye not believe him?”
(Because he had borne testimony to Christ, and had said, I am not the Christ, but He. “But if we shall say, Of men, we fear the people, lest they should stone us: for they held John as a prophet.” Afraid of stoning, but fearing more to confess the truth, they answered a lie to the Truth; and “wickedness imposed a lie upon itself.” For they said, “We know not.” And the Lord, because they shut the door against themselves, by professing ignorance of what they knew, did not open to them, because they did not knock.
For it is said, “Knock, and it shall be opened unto you.” Not only did these not knock that it might be opened to them; but, by denying that they knew, they barred that door against themselves. And the Lord says to them, Neither tell I you by what authority I do these things. And they were confounded by means of John; and in them were the words fulfilled, “I have ordained a lamp for mine anointed. His enemies will I clothe with shame.”
Source: Tractates on the Gospel of John (New Advent)