The Kingdom of God (or Kingdom of Heaven) is the sovereign reign and rule of God over all creation — the dynamic exercise of his kingly authority. It is not primarily a place but a reality: wherever God's will is perfectly obeyed, his Kingdom is present. The Kingdom was inaugurated in history by the coming of Jesus Christ ("the Kingdom of God is at hand" — Mark 1:15) and will be consummated at his return. Believers enter the Kingdom through repentance and new birth (John 3:3), living now as citizens of heaven and ambassadors of the King even while the full Kingdom awaits its final establishment. The Kingdom is both "already" (inaugurated) and "not yet" (consummated).
KING'DOM, n. The dominion of a king; the territory or country subject to a king or queen; a monarchical government. The Kingdom of God or of heaven, in the New Testament, signifies the kingdom of grace — the dispensation of grace and salvation by the gospel; or it denotes the church of Christ on earth; or it denotes the kingdom of glory in heaven.
The Social Gospel movement of the 19th–20th centuries redefined the Kingdom of God as a this-world utopia to be built by human effort — social justice programs, political activism, and moral reform. This strips the Kingdom of its supernatural character and makes it a synonym for progressive politics. On the other side, some Christians over-spiritualize the Kingdom, making it purely a future, heavenly realm, neglecting Christ's lordship over all of life now. Both errors miss the biblical "already/not yet" tension and Christ's present kingship over every domain.
Mark 1:15 — "The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel."
Matthew 6:33 — "But seek first the kingdom of God and his righteousness, and all these things will be added to you."
John 3:3 — "Jesus answered him, 'Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God.'"
Revelation 11:15 — "The kingdom of the world has become the kingdom of our Lord and of his Christ, and he shall reign forever and ever."
Colossians 1:13 — "He has delivered us from the domain of darkness and transferred us to the kingdom of his beloved Son."
G932 — βασιλεία (basileia): "kingdom, royal rule, reign" — the primary NT term for the Kingdom of God
G935 — βασιλεύς (basileus): "king" — the one whose reign constitutes the kingdom
H4438 — מַלְכוּת (malkuth): "kingdom, dominion, royal power" — used of God's eternal kingdom in Daniel and Psalms
"Jesus did not come merely to help people be better; he came to establish a Kingdom — and every knee will bow to its King (Philippians 2:10)."
"The Lord's Prayer is a Kingdom prayer: 'Your kingdom come, your will be done, on earth as it is in heaven' — we pray and labor for the King's rule to advance."
"Citizens of the Kingdom live by a different economy: the last shall be first, the servant is the greatest, and the cross is the path to the crown."