I think there is a difference between 'the kingdom of God' and 'the kingdom of heaven'. Obviously, the kingdom of heaven is in heaven.
I have found two ideas about the 'kingdom of God'. One, the kingdom of God are those of us who are on earth who are God's elect and chosen. We are ambassadors who share the Word and represent God.
Also, the kingdom of God is Jesus. Mark 1:14 says the kingdom of God has come near. Luke 9:27 says I tell you the truth, some who are standing here will not taste death before they see the kingdom of God.
2 Kings 19:15 Hezekiah prayed and said "Lord,... you are the God over all the kingdoms of the earth." We have to be careful that we do not reject the notion that the kingdom of God is on earth fighting for souls, because the kingdom does not look like what we think it should. Isn't that why the Sadducees rejected Jesus because he was not physically sitting on a throne like they thought he should be doing. They could not fathom any other king unless he was on a throne ruling on earth. The kingdom is kind of invisible in the sense that we could walk right by someone in the kingdom and not even know it. That is one of our biggest challenges...to get all the saints together in our town, in our church, in our community, on our street, in our homes to battle Satan.
Story in 2 Chronicles 13:1
Abijah, the king of Judah was preparing for battle with Jeroboam. Abijah had four hundred thousand (400,000) fighting men. Jeroboam had eight hundred thousand (800,000) able troops. Abijah went to Jeroboam and said, "Don't you know the Lord has given the kingship of Israel to David and his descendants forever.... You plan to resist the kingdom of David's descendants."
It is not a good idea to resist the kingdom of God. Jesus will come whether or not some believe it or not. Jesus will come get the people in his kingdom and take them to their inherited kingdom of heaven even if some don't believe it.
Blessings for now!