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 Luke 17:20-21  And when he was demanded of the Pharisees, when the kingdom of God should come, he answered them and said, The kingdom of God comes not with observation:
 Neither shall they say, See here! or, see there! for, behold, the kingdom of God is within you.
American King James Version×, 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 Matthew 12:14Then the Pharisees went out, and held a council against him, how they might destroy him.
American King James Version×, Mark 3:6 Mark 3:6And the Pharisees went forth, and straightway took counsel with the Herodians against him, how they might destroy him.
American King James Version×).
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 Matthew 23:15-17  Woe to you, scribes and Pharisees, hypocrites! for you compass sea and land to make one proselyte, and when he is made, you make him twofold more the child of hell than yourselves.
 Woe to you, you blind guides, which say, Whoever shall swear by the temple, it is nothing; but whoever shall swear by the gold of the temple, he is a debtor!
 You fools and blind: for whether is greater, the gold, or the temple that sanctifies the gold?
American King James Version×). 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.