You bring up the subject of kingdoms. Good! That is what Jesus' theme was as he preached throughout the land. "I must preach the kingdom of God to the other cities also, for I was sent for this purpose." (Luke 4:43, NASB)
He sent out his disciples to preach about God's kingdom also. "As you go, preach, saying, 'The kingdom of heaven is at hand.'" (Matthew 10:7)
Were they preaching about something in people's hearts? Or was that kingdom a REAL kingdom---a government with rulers and subjects? Is there reason to think that Jesus was talking about a real government? What do these scriptures suggest to us?
"For a child will be born to us, a son will be given to us; and the government will rest on his shoulders....There will be no end to the increase of his government or of peace, on the throne of David and over his kingdom." (Isaiah 9:6,7, NASB)
"In the days of those kings the God of heaven will set up a kingdom which will never be destroyed....it will crush and put an end to all these kingdoms, but it will itself endure forever." (Daniel 2:44, NASB)
Doesn't it look like God's Kingdom is more than just something in our hearts? When Jesus said to the religious leaders of his day, "The Kingdom is in your midst," couldn't he have meant that he, as the King designate, was among them at that moment?