Jesus responds by saying that he will answer their question if they will answer a question of his own first. "John's baptism: was it of God or merely human?" The religious leaders are politically calculating in their response. They know if they say from heaven then they should have believed John but if they say merely human then they will lose popularity with the crowds that regard John as a prophet. So they answer simply "We don't know." Jesus refuses to answer their question either.
If the story stopped at this point, then we might assume that Jesus was simply using the question about John to avoid the questioning of the religious leaders. But interestingly, Jesus doesn't drop his line of questioning when the elders and chief priests demonstrate their captivity to popular opinion. Instead, he tells a story of a Father who asks two of his children to go to work in his vineyard. The fist child says no but later goes anyway. The second child says yes but doesn't go. Jesus asks "Which of the two did the will of the Father?" The answer is clear. The first child did even though they said no initially. Jesus now brings the conversation back to John the baptist again.
"Truly I say to you, the tax collectors and the prostitutes go into the kingdom of God before you. For John came to you in the way of righteousness, and you did not believe him, but the tax collectors and the prostitutes believed him. And even when you saw it, you did not afterward change your minds and believe him."What is it that is so important about John's baptism to this question of Jesus' authority? For one thing, John was always pointing to Jesus. John's message was about one who was coming after him that was greater than him and who would baptize not with water but with the Holy Spirit and with fire. Therefore, a large part of what Jesus is saying in Matthew 21 is that if the religious leaders had accepted John's message then they would have accepted Jesus as well. Jesus says that is why the tax collectors and prostitutes go ahead of the chief priests and elder into Jesus' kingdom; they accepted John's baptism and the religious leaders did not. However, I think there is a little more going on here. In Matthew 3, we hear this about John's message and baptism
"But when he saw many of the pharisees and Sadducees coming to his baptism, he said to them 'You brood of vipers! Who warned you to flee from the wrath to come? Bear fruit in keeping with repentance. And do not presume to say to yourselves , 'We have Abraham as our father, for I tell you God is able from these stones to raise up children for Abraham. Even now the axe is laid to the root of the trees. Every tree therefore that does not bear good fruit is cut down and thrown into the fire."John's message is one of repentance and bearing fruit in keeping with that repentance. In other words, John is calling people away from the typical ways of doing religion (after all, he is dunking people in a dirty river out in the sticks away from the Temple and the holy city with all of its accepted religious institutions) and calling them to live lives that bear the fruit of God's love. Furthermore, he says that any tree which does not bear this fruit will be cut down.
What is fascinating about this is that in Matthew 21, right between Jesus' overturning of the tables in the Temple and his conversation with the religious leaders about his authority to do so, is a story about Jesus cursing a tree because it wasn't bearing fruit! Jesus actually wasn't evading the question of authority at all. He was pointing back to John's message of repentance because that was the key to understanding why Jesus exercised his authority as he did in the Temple. The Temple was a religious tree that wasn't bearing the fruit of God's love as God had meant for it too. So Jesus took the hatchet to it just as John had said he would.
We probably wouldn't be too happy with someone who came into our church and started turning everything upside down but the truth is that this is precisely what Jesus wants to do. There area all kinds of trees in our churches that aren't bearing the fruit of God's love and Jesus is more than willing to chop them down in order to make room for his house to be a house of prayer once again, for it to be a place where the blind and the lame are healed, a place where children sing "Hosanna to the Son of David!". Are we prepared to have Jesus take a hatchet to our church if that is what it takes to truly be his disciples?