Answering a question with a question is quite a common practice in a Jewish setting. But Jesus, of course, would not play with words. Evidently, behind the counter-question He asks lies something important, something on which His interlocutors' very ability to understand Him depends. And it apparently concerns John the Baptist. Jesus' interlocutors begin thinking hard: what can they say so as not to lose out? They clearly did not want to acknowledge John as a prophet; they probably still doubted his prophetic calling. On the other hand, the people saw John precisely as a prophet, while the Pharisees always played the role of spiritual leaders and teachers, so it was obviously not in their interest to come into conflict with public opinion. Hence the answer in the finest tradition of political manoeuvring: no comment, “we do not know,” although, of course, given the stature of John's personality, it was entirely impossible in Judea not to “know” or have an opinion about his activity.
And Jesus, of course, understands all this perfectly. In turn, He refuses to answer the question asked by the Pharisees. This is not because He wants to conceal anything, but because it is obvious to Him that answering is pointless: these people will not hear Him anyway; they want a predetermined answer that would confirm their own ideas. If the answer is different, it, just like the question that has just been asked, will remain without comment and without consequence.
In essence, it is not so important exactly what answer Jesus would have heard to His question. The main thing is that an answer should be given, an honest, sincere, and unambiguous one, whatever it might be. Any such step could have become the beginning of further discussion, perhaps a very tense one. But evading it immediately and completely cuts off every possibility of further communication.
The point is not even that Jesus' interlocutors are dissembling before Him; after all, it is not particularly difficult for Him to understand what they really think and mean. The point is that they are dissembling first of all before themselves: about the fact that what they need is not an answer but confirmation, and that what interests them is not the truth but their own preservation as spiritual leaders. This is a genuine spiritual dead end, and all discussion is useless here.
