What does Luke 17:21 mean?

The Pharisees have asked when the kingdom of God is coming (Luke 17:20). A major portion of Luke's record (Luke 9:51—19:27) is about God's kingdom, yet the Pharisees refuse to understand (John 5:39–40). They emphasize the glorious vision of the Son of Man who will be "given dominion and glory and a kingdom, that all peoples, nations, and languages should serve him; his dominion is an everlasting dominion, which shall not pass away, and his kingdom one that shall not be destroyed" (Daniel 7:14). They're primarily looking for a military leader who will drive the Romans out and return independence to the Jews.

That will certainly happen at Jesus' second coming, but the Pharisees are part of His first arrival on earth. At the time depicted here, the kingdom of God is all around them. It's in the healings Jesus performs (Luke 4:40), the way His disciples are empowered to cast out demons (Luke 10:17), in the way a person gives a drink of water to someone who is thirsty (Mark 9:41), and in Jesus' example of humble service (John 13:1–5).

The Samaritan leper understood this. When he realized he was cleansed, he returned to Jesus and declared that Jesus healed him to the glory of God. In response, it appears this otherwise-rejected man (John 4:7–9) became a permanent citizen of God's kingdom (Luke 17:11–19).

Jesus goes on to teach His disciples how they can recognize the fulfillment of God's kingdom at Jesus' second coming. There won't be any doubt; it will be as obvious as flashing lightning across the sky that all can see and hear. First, however, He must be betrayed and sent to the cross (Luke 17:22–25).