The thoughts of kings and kingdoms are foreign to our 21st-century mindset. The very mention of them brings to mind fairy tales and medieval legends. Our view of a kingdom is romanticized and very separated from our day-to-day life.
But in the ancient world, the king was supreme. His word was absolute and uncontested. The citizens would thrive under a good king or chafe under a bad king, who would rule until his death or defeat (by a more powerful king).
When God established the nation of Israel and called them to be His own people, He set up a government where God Himself would rule as their perfect king (see Exodus-Deuteronomy). But the people rejected God’s rule and demanded a king of their own (1 Samuel 8:4-9). Throughout the rest of Old Testament History, the fortunes of God’s people rose and fell based on the character of their kings.
But God had greater plans for His people. Every book of the Old Testament promised that a great king would come. He would save mankind from the curse of sin, and lead Israel in a victorious, perfect kingdom. He was the Promised One, the Anointed One, or in Hebrew, The Messiah (see here).
The Old Testament prophets also wrote about His kingdom. Daniel saw the empires of mankind destroyed by the kingdom of God that would never end (Daniel 2:44, 7:27, see here). Isaiah saw the kingdom of God as a kingdom of peace and safety, where the wolf would dwell with the lamb (Isaiah 11).
This kingdom of God was a real promise that Jews anticipated. As they were dominated by the world empires — Babylon, Persia, Greece, and Rome — they looked forward to the day when their Messiah would come and free them, restoring peace to the world. When John the Baptist came announcing that the kingdom of heaven was at hand, the people came by the thousands. They were ready to be free!
Jesus followed John with the same message: the kingdom of heaven was at hand1. But Jesus never came with military force. He never drove out the Romans, nor did He oppose any conquering army. He had no political power. Yet Jesus spent most of His ministry teaching about His kingdom. So what is His kingdom? What is the kingdom of heaven?
One of the main points of Matthew’s gospel account is to tell about the kingdom of God. From the books of Matthew, Mark, Luke, and John, we see the following about the kingdom of God:
The kingdom of God is both physical and spiritual. The Old Testament prophets predicted a concrete, physical kingdom, not an abstract spiritual state. Daniel predicted a real government to destroy the empire of Rome. Isaiah predicted a real world of peace, with real wolves and real lambs living together.
The kingdom is physical, yet there is a much greater spiritual aspect to God’s kingdom. You could not be part of the kingdom until you first turned from your disobedience to God. Both John the Baptist and Jesus preached to “Repent, for the kingdom of heaven is at hand!” (Matthew 3:1-2, Matthew 4:17). Jesus met with Nicodemus at the beginning of His ministry and told him, “unless one is born again he cannot see the kingdom of God” (John 3:3). God’s rule in the spirit world is even greater than any physical kingdom, and we cannot even see it unless we are born again (see here).
When Jesus came to earth, He offered the physical kingdom to His people. He went through the towns of Galilee, spreading the message that the kingdom was at hand (Matthew 4:23-25, see here). He then appointed twelve disciples to go out (in pairs) with the same message (Matthew 10). Later in His ministry, He sent an additional seventy-two (72) to carry the same message throughout Judea (Luke 10:1-12).
But the people did not believe, therefore Jesus turned away from them (Matthew 21:43). Matthew 12-13 shows a united rejection against Jesus. Afterward, Jesus began to withdraw from the people, focusing on His disciples, and using parables to teach only the believers, hiding the message from unbelievers (Matthew 13:10-15).
By the end of Jesus’ ministry, He was focused on the spiritual kingdom. At his trial, Jesus told Pilate, “My kingdom is not of this world” (John 18:36).
This greater part of the Kingdom of God, the spiritual kingdom, has continued to grow since the time of Jesus Christ on earth. Jesus taught about the Kingdom of God after His resurrection (Acts 1:3), and both Philip and Paul preached about the Kingdom of God (Acts 8:12, 14:22, 28:23). The New Testament epistles teach of the Kingdom of God as both a present reality and a future inheritance (Romans 14:17; 1 Corinthians 4:20, 6:9-10, 15:50; Galatians 5:19-21; Ephesians 5:4-5; Colossians 4:11; 1 Thessalonians 2:12; 2 Timothy 4:1; James 2:5).
Finally, we are given hope that Jesus will return and set up His kingdom over the earth at the end (Revelation 12:10, 20:4-5). Satan will be defeated and Jesus Christ will reign over the earth for a thousand years!
 Matthew uses the term, “kingdom of heaven”, while Mark, Luke, and John use the term, “kingdom of God”. Most Bible scholars consider these two terms synonymous. There may be a nuance related to the context or the audience, but the meaning is essentially the same.