Theophylact of Ohrid

For that He might show that they had always rebelled against the Holy Spirit, and that besides Isaiah, whom they remembered not, they had refused to believe John whom they had lately seen; He now in his turn puts the question to them, proving that if so great a Prophet as John who was accounted greatest among them had been disbelieved when he testified of Him, they would in no wise believe Him, answering by what authority He did this.

Catena Aurea by Aquinas ↗

Bede

But answering, he said to them, "I will also ask you one thing. Answer me. Was the baptism of John from heaven or from men?" The Lord could have confuted the slander of the tempters with an open answer, but prudently he asks so that by their own silence or response, they might be condemned.

On the Gospel of Luke ↗

Bonaventure

Second, as to the sagacity of Christ in resisting, it is added: But Jesus answering said to them: I also will ask you one word. When he should have responded, he asked a question—not because he did not know, but because they are better bound by a question posed than by a direct answer. …

Commentary on Luke, Chapter 20 ↗