Virtually any person who has read the New Testament would agree that the kingdom of God was at the center of the message preached by Jesus and the apostles. In Matthew, Jesus' sayings refer to the kingdom no less than 45 times, and all four Gospels contain important mentions of this notion of the kingdom of God.
Both Mark and Matthew summarize Jesus' message along the lines, "The kingdom of God is at hand; repent and believe" (Mark 1:15; Matthew 4:17). In Luke 4:43, Jesus declares that the purpose for which he was sent was to "preach the good news of the kingdom of God". Acts records the kingdom of God as the primary focus of Jesus' discussions with the apostles after his resurrection and prior to his ascension (Acts 1:3). The kingdom of God also features prominently in summary statements about apostolic preaching in Acts (e.g. 8:12; 28:31). It is also mentioned over 20 times in the rest of the New Testament.
While most are agreed on the centrality of the kingdom of God in Jesus' and the apostles' teaching, there is no such agreement on what the kingdom of God actually means. Baker's Evangelical Dictionary of Biblical Theology mentions four different interpretations. These are (1) the Political Kingdom (in which Jesus made a failed attempt to establish a political kingdom in rebellion against Rome), (2) the Spiritual Kingdom (in which the kingdom refers to God's rule in the individual's heart), (3) the Consistent or Future Kingdom (in which a supernatural kingdom which does not yet exist will be established after the Second Coming of Christ), and (4) the Realized or Present Kingdom (in which Jesus brought the kingdom at his first coming and fully established it through the church).
While interpretations (1) and (2) no longer have a following among any but the most liberal of Bible scholars, today's Evangelical Christianity is, in some instances, polarized between (3) and (4). Christadelphians (if I may be permitted to lump them in with Evangelical Christianity) have traditionally been firmly at the (3) end of the pole. The kingdom of God is yet future, and any insinuation that it may exist presently is taken as false doctrine, full stop. For instance, the 1877 Christadelphian Statement of Faith contained eight clauses describing the kingdom of God, all of which referred strictly to the future (although it is stated that this future kingdom will be a recapitulation of the past Davidic kingdom). Meanwhile, among the Doctrines to be Rejected is the idea that the kingdom of God is the church.
At the other extreme, in some Evangelical churches, there is an extreme at the (4) end. One could attend such churches for months and only hear references to the kingdom as a present reality. Talk of a future kingdom, for such believers, suggests a detachment from the Lord's powerful present work among his people. Besides, if such a kingdom was going to come, surely it would have come by now!
As is often the case, both extremes are wrong. A comprehensive biblical doctrine of the kingdom of God must incorporate both (3) and (4). There are passages which refer to the kingdom of God as a future reality only to be accomplished at the end of the age (Matt. 8:11; Matt. 25:34; Luke 19:11ff; Luke 22:18; Gal. 5:21; 2 Tim. 4:1; 2 Pet. 1:11), and there are passages which refer to the kingdom of God as having already arrived in the first century (Matt. 4:17; Matt. 13:41; Matt. 21:43; Luke 10:9-11; Luke 17:20-21; Col. 1:13; Rev. 1:9).
This balanced approach, in which the kingdom has been inaugurated but not consummated, is sometimes referred to as inaugurated eschatology. It is necessary to hold in tension the paradox that the kingdom of God is both now and not yet. Only then can we avoid the extremes of the one whose anticipation of future events distracts him from the Lord's presence and power in the church today, and the one whose focus on present spiritual realities has left him with no sense of the approaching conclusion of history.