In the Scriptures the kingdom of God is conceived of in different ways. On the one hand, God rules over all things, and always has. “The Lord has established his throne in the heavens, and his kingdom rules over all” (Ps. 103:19). “His dominion is an everlasting dominion, and his kingdom endures from generation to generation” (Dan. ).
On the other hand, the kingdom of God is conceived of as having a beginning in history, at the time of Christ.
God’s kingdom and rule are eternal; but with the fall, the human race entered into a state of rebellion against his authority. Thus God seeks to reassert his crown rights and reduce the rebels to a willing and whole-hearted obedience…or else destroy them if they remain impenitent. This is the cosmic battle, of which we are all a part. We are all enlisted on one side or the other. It is a battle of two kingdoms: the kingdom of heaven, governed by God vs. the kingdom of the world, governed by the devil.
Jesus’ incarnation was an invasion of the kingdom of God into hostile territory—where God himself, in the person of Jesus Christ, came to earth to reclaim territory held by his enemies. In this sense, we can think of the kingdom of God as having a beginning at the time of Christ.
In fact, I think it’s helpful to think of the kingdom as being definitively established at the time of Christ; progressively established through history; and finally established at the consummation of all things, when Christ comes again.
The definitive establishment of the kingdom of God took place in Jesus’ day. The kingdom came when Jesus came. He began his ministry by saying, “The time is fulfilled, the kingdom of God is at hand” (Mk. 1:14-15; Matt. 4:17). In other words, “The time has come; the time is now; the kingdom is here.” During his ministry Jesus told the Pharisees that his casting out of demons was proof that the kingdom of God had come (Matt. ). He also said that the kingdom was in their midst (Lk. ).
The point is that, Jesus, in his ministry, life, death, resurrection, and ascension definitively established the kingdom of God. His ascension and seating at the right hand of God the Father was his formal inauguration as King (Acts -36).
But then the kingdom of God grows and expands in history as more and more people come to submit to the Lordship (Kingship) of Jesus. When someone is “born again” (Jn. 3:3, 5) he is “delivered...from the domain of darkness and transferred...to the kingdom of [God’s] beloved Son” (Col. 1:13). Jesus told the parables of the mustard tree and of the leaven to illustrate the progressive and expansive nature of the kingdom (Matt. -32, 33).
The kingdom of God will continue to grow until it is all-encompassing. “He [Christ] must reign until he [God] has put all his enemies under his feet” (1 Cor. ). This refers to a time when all men and nations are brought to obedience—when all opposition to the rule of God is brought to an end. “Then comes the end,” we are told, “when he delivers the kingdom to God the Father after destroying every rule and every authority and power” (v. 24).
The kingdom of God, then, is a present reality; although we don’t experience it in its fullness. When we pray in the Lord’s Prayer, “Thy kingdom come”, we are praying for the progressive expansion of the rule of God over the hearts and lives of men. It’s a prayer that non-Christians would be converted (in this sense it is an evangelistic prayer), and a prayer that the saints would grow up into the full stature of Christian maturity by ordering their lives according to the rule laid down in God’s word.