Answer: The verse regarding God's kingdom being "within you" is taken from the King James translation of Luke 17:21. It was given by Jesus as he and his disciples traveled south through Galilee and Samaria on their way to Jerusalem (Luke 17:11). Just before he made the statement in question he healed ten lepers, only one of which came back to thank him for what he did (verses 12 - 19).
After the healing the Bible records the Pharisees demanding he tell them when the Kingdom would come (Luke 17:20). Jesus, near the end of his ministry, knew that the Pharisees and most of the religious leaders hated him and his message. He knew they wanted him dead. In fact, sometime prior to their request, he pointedly told them that their true spiritual father was Satan the devil (John 8:40 - 44)!
Jesus' answer to the Pharisees' demand of knowledge regarding when righteousness would rule the earth was swift. He informed the self-righteous that, "Neither shall they say, 'Behold, it is here!' Or, 'Behold, it is there!' For behold, the kingdom of God is standing IN THE MIDST OF you" (Luke 17:20 - 21, HBFV).
Kingdom in their hearts?
Was Jesus now informing the same unconverted Pharisees who sought to kill him (John 8:28, 37, 11:53) that they had eternal life 'within their hearts?' No, he was not! Part of the problem in misunderstanding his statement lies with the erroneous KJV translation of the Greek word entos (Strong's Concordance # G1787) as "within." Other Bibles, that are more accurate, translate the word as "in the midst of" or "among."
It was in the midst of, or among, the Pharisees in the person of Jesus that God's Kingdom was manifested to them. Christ, before this encounter, equated his presence and power with the rule his Father gave him when he stated the following.
And if I by Beelzebub cast out demons, by whom do your sons cast them out? Because of this, they shall be your judges. But if I . . . cast out demons, then the KINGDOM OF GOD has come upon you (Luke 11:19 - 20).
Notice what happens when Christ returns to the earth the second time. The book of Revelation states the following.
Then the seventh angel sounded his trumpet; and there were great voices in heaven, saying, "The kingdoms of THIS WORLD have become the kingdoms of our Lord and His Christ, and He shall reign . . . (Revelation 11:15, HBFV)!
The seventh angel heralds the coming Kingdom of Heaven. It will have a King (1Timothy 6:15, Revelation 17:14, 19:16) and territory (the entire planet). It will contain subjects (all the people who survived Satan's rule) and laws to govern with (the Ten Commandments).
The Kingdom of God is not within each human being. Jesus was not giving a pretty sentiment about moral improvement in the hearts of man or the automatic placement of eternal life inside a human (e.g. immortal soul). He, as its King, was discussing a soon coming new government coming to save man from himself!