Asked by the Pharisees when the kingdom of God would come, he said in reply, "The coming of the kingdom of God cannot be observed, and no one will announce, 'Look, here it is,' or, 'There it is.' For behold, the kingdom of God is among you." Then he said to his disciples, "The days will come when you will long to see one of the days of the Son of Man, but you will not see it. There will be those who will say to you, 'Look, there he is,' (or) 'Look, here he is.' Do not go off, do not run in pursuit. For just as lightning flashes and lights up the sky from one side to the other, so will the Son of Man be (in his day). But first he must suffer greatly and be rejected by this generation. (Lk 17:20-25)
The question raised by the Pharisees only shows the view of the Pharisees with regard to the kingdom. They thought that the kingdom would be within a time frame and it would be preceded by this and that. It should be visible to human eyes and observable in human conditions etc.
Jesus’ answer gives a different picture about the kingdom. He says that the kingdom is within you. Some thought that this would mean a spiritual reality. It is a private and a personal relationship with God. But kingdom is something in public. There are people who say the kingdom is present but secret, hidden, waiting to be discovered.
But Jesus really means is the kingdom is within your reach. It is with in your grasp. You have got to do something about it. One has to make a decision to be in the kingdom. The kingdom challenges us to make a decision to believe in Jesus, trust him and follow him.