The Kingdom of God is a moral kingdom that God is seeking to establish in the hearts of men. It doesn’t have a specific beginning, although technically it begins with Adam. It will have its final culmination in the Millennial reign of Christ, when “every knee shall bow and every tongue confess that Jesus Christ is Lord to the glory of God the Father.” Technically, the Kingdom of God will never end, because His saints will be eternally subject to Him.
A moral kingdom. Romans 14:17 tells us that the kingdom of God does not consist of physical things, but of moral qualities that reflect the character of God. “For the kingdom of God is not meat and drink; but righteousness, and peace, and joy in the Holy Ghost.”
Compared to the Kingdom of Heaven. The kingdom of God is a broad term found in all four gospels and some of the epistles, while the kingdom of heaven is a specific dispensational term only found in Matthew’s gospel. The “kingdom of heaven” refers that state of things where the heavens rule now that Jesus is cast out from the earth. The kingdom of heaven did not “come” until Christ returned to the Father. But the kingdom of God was already “come” unto them (Matt. 12:28) in the Person of the King. Therefore, when you get the kingdom of heaven you also have the kingdom of God there, but certain things are said of the kingdom of God that could not be said of the kingdom of heaven.