The kingdom of God can be defined as the king himself, king Jesus. We are his people, his body, and so co-rulers. We will, as Paul says, judge the angels. Yet the full expression of the kingdom in all its magnificence will come when Jesus returns to judge the quick and the dead.
Over the last few hundred years, Christians have debated the nature and timing of the kingdom of God. Some argued that Jesus was a failed prophet since the kingdom did not come as he expected. Others argued that the kingdom was delayed due to Israel’s rejection of Jesus. Still others found ways to understand the kingdom apart from either extreme.
From an historical angle, the debate is fascinating since it did not seem to greatly disturb earlier Christians. For example, John Scotus Eriugena (9th ce.) maintained the presence of the kingdom now yet with its full manifestation appearing at a later time.
I think we can be even more precise than John Eriugena since Scripture speaks on this matter. The kingdom of God in its political and geographical reach lies in the future after the second coming, while the presence of the kingdom exists in the people of God because they are in Jesus.
Millennial theology largely relies on John’s writings, especially Revelation 20. For this reason, we should start with John. According to the apostle, Jesus “made us a kingdom” (ἐποίησεν ἡμᾶς βασιλείαν; Rev 1:6). This “made us” signifies a past realities that we now experience.
In order to clarify exactly what this means, John specifies that Christians are partners in the kingdom “in Jesus” (ἐν Ἰησοῦ,; Rev 1:9). As king, being in him, we are the kingdom of God on earth. The “in” here demonstrates that the kingdom of God is here in Christ whose body we are. This agrees with Jesus’s proclamation to the religion leaders in Luke: “the kingdom of God is in your midst” (Luke 17:21).