There are many folks today who believe the church Jesus established in the New Testament and the Kingdom of God prophesied about in the Old Testament are two different things. One of the premier scriptures that show they are the same thing is Isaiah 2:2-4. It says,  "And it shall come to pass in the latter days, that the mountain of Jehovah's house shall be established on the top of the mountains, and shall be exalted above the hills; and all nations shall flow unto it.  And many peoples shall go and say, Come ye, and let us go up to the mountain of Jehovah, to the house of the God of Jacob; and he will teach us of his ways, and we will walk in his paths: for out of Zion shall go forth the law, and the word of Jehovah from Jerusalem.  And he will judge between the nations, and will decide concerning many peoples; and they shall beat their swords into plowshares, and their spears into pruning-hooks; nation shall not lift up sword against nation, neither shall they learn war any more.” 

Notice that it calls attention to the "latter days” which began on Pentecost following the resurrection of Christ (Acts 2:17). Also the "mountain of Jehovah’s house” was to be established in Jerusalem, and would be open to "all nations,” not just Israel. His Word or His Law would be taught beginning at Jerusalem, reminding us of what Jesus told the apostles in Acts 1:8. The peaceful nature of Christians and the kingdom of God is described in Matthew 26:52; John 18:36; and 2 Corinthians 10:3-5. All parts of this marvelous prophecy written 700 or more years before Christ, began to be fulfilled on Pentecost following the resurrection of Christ (Acts 2). This also proves the kingdom is the church since this is the birthday of the Lord’s church (Acts 2:47; 5:11). Thus, this passage conclusively proves that the Lord’s church and the kingdom of God are one in the same!
The book of Zechariah is often quoted by those who believe that the kingdom of God is yet to be built, but this book also shows conclusively that it refers to the church of the Lord that began on Pentecost (Acts 2)! Note the plain prophecies fulfilled by that time: Zechariah 6:12-13 says that a man called the "Branch” would "build” the temple of the Lord, He would bear the "glory” and sit and rule "upon his throne” and be a "priest upon his throne.” Now look at how these are already fulfilled in the New Testament. Jesus said He would "build his church” (Matthew 16:18). He is also depicted by the apostle Peter as now sitting and reigning on His throne in heaven (Acts 2:30-34). Jesus is also described by the apostle Paul as the "blessed and only Potentate, the King of kings and Lord of lords” (1 Timothy 6:15). The writer of Hebrews (3:1) describes Jesus as our "Apostle and High Priest of our confession.” All of these are clearly fulfilled in the New Testament, not some time yet in the future. However, Zechariah is not done. Chapter 9:9 describes Jesus coming: just, lowly, bringing salvation and "riding upon an ass, even upon a colt the foal of an ass” (see Matthew 21:5; John 12:13-15). These were obviously fulfilled in Jesus’ Triumphal Entry. But Zechariah isn’t through. Judas’ reward for betraying the Lord and its eventual use in buying the potter’s field is prophesied in 11:12-13 and fulfilled in Matthew 27:1-10. In Zechariah 12:10 and 13:1 the piercing of Jesus’ side in His crucifixion and the marvelous results the shedding of His blood would bring are vividly depicted. They are fulfilled in (John 19:34; Romans 5:9; Hebrews 9:14). Remember all these were written between 480 and 520 years before Jesus was born! 

Not only do these many prophecies (and dozens more could be added) that were written so many hundred years before Christ show the awesome foreknowledge of God in revealing these things, and the inspiration of the Bible, but they show conclusively that this great Kingdom foretold by the prophets is the church of our Lord and has been in existence for almost two thousand years!
Ed Rodgers
Muskogee, OK