2 Therefore was John sought for to bear witness to the Truth; and you have heard what He said; “You came unto John; he was a burning and a shining lamp, and you were willing for a season to rejoice in his light.” This lamp was prepared for their confusion, for of this was it said so long time before in the Psalms, “I have prepared a lamp for Mine Anointed.” What! A lamp for the Sun! “His enemies will I clothe with confusion: but upon Himself shall my sanctification flourish.” And hence they were in a certain place confounded by means of this very John, when the Jews said to the Lord, “By what authority do You do these things?
Tell us.” To whom He answered, “Do ye tell Me too, The baptism of John, was it from heaven, or of men?” They heard, and held their peace. For they thought at once with themselves. “If we shall say, Of men: the people will stone us; for they hold John as a prophet. If we shall say, From heaven; He will say to us, Why then have ye not believed him?” For John bore witness to Christ. So straitened in their hearts by their own questions, and taken in their own snares, they answered, “We do not know.”
What else could the voice of darkness be? It is right indeed for a man when he does not know, to say, “I know not.” But when he does know, and says, “I know not;” he is a witness against himself. Now they knew well John's excellency, and that his baptism was from heaven; but they were unwilling to acquiesce in Him to whom John bore witness. But when they said, “We do not know;” Jesus answered them. “Neither will I tell you by what authority I do these things.” And they were confounded; and so was fulfilled, “I have prepared a lamp for Mine Anointed, His enemies will I clothe with confusion.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)