The kingdom of God is referenced often in the gospels (e.g., Mark 1:15; 10:15; 15:43; Luke 17:20) and other places in the New Testament (e.g., Acts 28:31; Romans 14:17; 1 Corinthians 15:50). The kingdom of God is synonymous with the kingdom of heaven. The concept of the kingdom of God takes on various shades of meaning in different passages of Scripture.
Broadly speaking, the kingdom of God is the rule of an eternal, sovereign God over all the universe. Several passages of Scripture show that God is the undeniable Monarch of all creation: “The LORD has established his throne in heaven, and his kingdom rules over all” (Psalm 103:19). And, as King Nebuchadnezzar declared, “His kingdom is an eternal kingdom” (Daniel 4:3). Every authority that exists has been established by God (Romans 13:1). So, in one sense, the kingdom of God incorporates everything that is.
More narrowly, the kingdom of God is a spiritual rule over the hearts and lives of those who willingly submit to God’s authority. Those who defy God’s authority and refuse to submit to Him are not part of the kingdom of God; in contrast, those who acknowledge the lordship of Christ and gladly surrender to God’s rule in their hearts are part of the kingdom of God. In this sense, the kingdom of God is spiritual—Jesus said His kingdom was not of this world (John 18:36), and He preached that repentance is necessary to be a part of the kingdom of God (Matthew 4:17). That the kingdom of God can be equated with the sphere of salvation is evident in John 3:5–7, where Jesus says the kingdom of God must be entered into by being born again. See also 1 Corinthians 6:9.
There is another sense in which the kingdom of God is used in Scripture: the literal rule of Christ on the earth during the millennium. Daniel said that “the God of heaven will set up a kingdom that will never be destroyed” (Daniel 2:44; cf. 7:13–14), and many of the other prophets predicted the same thing (e.g., Obadiah 1:21; Habakkuk 2:14; Micah 4:2; Zechariah 14:9). Some theologians refer to the future, open manifestation of the kingdom of God as the “kingdom of glory” and the present, hidden manifestation of the kingdom of God as the “kingdom of grace.” But both manifestations are connected; Christ has set up His spiritual reign in the church on earth, and He will one day set up His physical reign in Jerusalem.
The kingdom of God has several aspects. The Lord is the Sovereign of the universe, and so in that sense His kingdom is universal (1 Timothy 6:15). At the same time, the kingdom of God involves repentance and the new birth, as God rules in the hearts of His children in this world in preparation for the next. The work begun on earth will find its consummation in heaven (see Philippians 1:6).