Many people believe Jesus Christ taught that the Kingdom of God is something that exists only in the hearts and minds of believers. They base this on Luke 17:20-21, which says: "Now when He was asked by the Pharisees when the kingdom of God would come, He answered them and said, 'The kingdom of God does not come with observation; nor will they say, 'See here!' or 'See there!' For indeed, the kingdom of God is within you.'"
Such assumptions are incorrect for several reasons. The Greek word entos, translated "within," is better translated "in the midst of" (Vine's Complete Expository Dictionary of Old and New Testament Words, "Within"). Several translations, including the Revised Standard Version, Jerusalem Bible and New English Bible, make this clear. Jesus Christ could not have been telling the Pharisees here that God's Kingdom was something that existed within their hearts or minds—after all, they wanted to destroy Him (Matthew 12:14, Mark 3:6).
Instead, in this passage Christ was pointing out the paradox that the Pharisees did not have the spiritual discernment to recognize that the message of the Kingdom of God was at hand or being offered to them (Matthew 23:15-17). To punctuate this point, Jesus, referring to Himself, said "the kingdom of God is among you" or "in your midst." The spiritually blind Pharisees did not recognize Jesus as the divine Representative of that Kingdom.
Rather than telling the Pharisees that the Kingdom of God was something in their hearts, Jesus Christ warned them that they were so spiritually blind they couldn't recognize the very personification of that Kingdom in Him. There is no basis in this passage for believing the Kingdom of God resides in one's heart.