Jesus answered, ‘My kingdom is not of this world. If my kingdom were of this world, my servants would have been fighting, that I might not be delivered over to the Jews. But my kingdom is not from the world.’ John 18:36
Do a word search on “kingdom” in the New Testament, and you’ll be surprised at how often it pops up. That’s because the kingdom of God was a very important topic in first-century A.D. Christianity, and rightly so. Jesus preached about it throughout Galilee, declaring that it was at hand (Mark 1:14-15; cf. Matt. 4:23; 9:35; Luke 4:43; 8:1). It was the theme of both the Sermon on the Mount (Matt. 4:23; 5:1-3, 10, 19-20; 6:10; 7:21-27) and the majority of his parables (cf. Matt. 13:44-46). He sent his disciples out to preach about it (Luke 9:1-2, 59-60; 10:1, 9-11). He preached about it after he died and was resurrected (Acts 1:3). After he ascended, his followers preached about it (Acts 8:12; 14:21-22; 19:8; 20:25; 28:23, 30-31) and wrote about it (cf. 1 Thess. 2:12; 2 Thess. 1:5).
As seen above, while on trial Jesus said to Pilate that his kingdom “is not of this world” (John 18:36). Other comments he made give us an inclination about the nature of his kingdom. When a scribe complimented Jesus’ answer about what is the greatest commandment, Jesus said to him, “You are not far from the kingdom of God” (Mark 12:28-34), indicating that the scribe’s understanding of the Scriptures put him in close proximity to the kingdom. In a way that meant he was close to the geographical borders of a kingdom? No, for Jesus’ kingdom isn’t of this world. Thus, Jesus was showing that the kingdom is spiritual in nature (cf. Luke 17:20-21).
Paul’s told the Colossian Christians that God had transferred them into the kingdom of his Son (Col. 1:13). The writer of Hebrews told the Hebrew Christians that they had received a kingdom (Heb. 12:28). John said to the churches of Asia Minor that Jesus had made them a kingdom and that he was partners with them in the kingdom (Rev. 1:6, 9). That tells me that the kingdom of God is the church, Christians are citizens of that kingdom, and Jesus is its King.
Paul also said that at the end when Christ returns he will give the kingdom back to his Father after destroying all other rules, authorities, powers, and after having put his enemies under his feet, including death (1 Cor. 15:24-28). On that day his angels will take out of his kingdom all who unrepentantly disobey God and cast them into hell, after which the righteous in the kingdom “will shine like the sun” (Matt. 13:41-43). Christian, what kind of citizen of God’s kingdom are you? Are you submissive to the King in all things, or are you unrepentant in your sin? Are you even a part of his spiritual kingdom to begin with? If not, do what they did in the New Testament (Acts 8:12). Repent of your sins, confess your faith in Christ, and be baptized into his body (Acts 2:38; Rom. 10:9-10; 1 Cor. 12:13), which is his church (Col. 1:18), which is his kingdom (Col. 1:13).