This short post should help define the term. A simple five-step explanation of the Kingdom is included, as well.
The kingdom is God’s right to rule and reign. It is his realm that is controlled by him.
First, we can see these realms more clearly with contrasts. The kingdoms of humanity are controlled by men and women. The spiritual kingdom of darkness, invisible to the unaided eye, is under Satan’s control. And the kingdoms of man and Satan overlap. But God’s kingdom cannot be hindered by powerful humanity or Satan. God is large and in charge. God’s kingdom will sweep aside the inferior kingdoms at the Second Coming (Rev. 20-21).
Second, there is an aspect of the kingdom of God that is gradually descending or being revealed to humanity. It is called the “already and not yet,” combined. When the gospel is preached, the kingdom is already here. Wherever the gospel is not preached, the kingdom is in the “not yet” phase. Around the globe, the kingdom is already here in part, but not yet here in its fullness.
Third, proclaiming “Jesus is Lord” in the Roman empire was like saying “Caesar is not Lord.” It was like saying, “The true king and his kingdom are here!” It may be true that the word kingdom is not found in the epistles as often as in the synoptic Gospels (Matthew, Mark, and Luke), but the reality is behind the teachings of the epistles. The title Lord Jesus or the Lord is used often enough.
Fourth and finally, let’s boil it down. Kingdom means that God has the right to rule over our lives. It is not abstract, but is manifested in people. The global church is not the same as the kingdom of God, but the kingdom creates the church. If we equate the two, we blur essential distinctions. For example, we might believe, wrongly, that as the church gets bigger, then it has the right to rule over the world systems.
The Kingdom of God works out like this in five simple steps.
Individuals Christians can get involved as deeply in the world systems as God calls them, say, in education or politics or business. But the church as an institution should not gobble up these systems of the world (though they may certainly create schools that honor God and his kingdom). That will happen only when Christ sweeps them aside at his Second Coming. Before then, the best way to introduce the kingdom of God to the world and see it expand is by preaching the gospel.