Many today teach that the kingdom has not yet been established.  The teaching is that Jesus will return to set up a physical kingdom that will last for a thousand years in Jerusalem.  However, Jesus will not return to begin a physical kingdom, in this world, according to scripture (John 18:36).

Jesus is currently ruling as king in His spiritual kingdom, the church, which began on Pentecost when Peter told the Jews what they must do to be saved (Acts 2:37-38).

The kingdom was foretold by the prophets.  In Isaiah 2:2-3 it states, "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."

Notice these pertinent things:  (1) It would be "in the last days."  (2)  The "mountain of the Lord's house" would be established. (3)  It would include "all nations."  (4) It would go forth "from Jerusalem."
        
         The Kingdom Preached in Preparation
 
"In those days came John the Baptist, preaching in the wilderness of Judea, and saying, Repent ye for the kingdom of heaven is at hand: (Matt. 3:1-2)

Notice that John points to the future - it was "at hand" but not yet present.  John was not in the kingdom, for Jesus said, "The least in the kingdom is greater than he" (Matthew 4:17).  So John spoke of that which should come, in the future.  Jesus began His personal ministry on earth, saying, "Repent: for the kingdom of God is at hand" (Matthew 4:17).  Later He sent forth the twelve and the seventy, and they preached the same message:  "The kingdom is at hand."  It was yet in the future, but approaching.  Jesus told the apostles, "There are some of them that stand here that shall not taste of death, till they see the kingdom come with power" (Mark 9:1)  Now we see the coming of the kingdom pinpointed.  At His ascension, when they asked of the kingdom, Jesus replied, "Ye shall receive power when the Holy Spirit is come upon you" (Acts 1:8).

Isaiah promised, "The mountain of the Lord's house shall be established in the top of the mountains."  But of what was he speaking?  What is the Lord's house? The apostle Paul, by the same Spirit, gave us the meaning:  "The house of God, which is the church of the living God" (1 Timothy 3:15).  So the Lord's house is His church.
       
          The Fulfillment of These Prophecies
 
It is in Acts 2 that we find the fulfillment of the promises we have noted above.  First of all, the apostles were in the right place: Jerusalem, as both Isaiah and Micah had foretold.  Jesus also had revealed this:  "Behold, I send forth the promise of my Father upon you:  but tarry ye in the city, until ye be clothed with power from on high (Luke 24:49).  The first gospel sermon was preached in Jerusalem on the day of Pentecost, and then to Judea, Samaria, and the world (Acts 1:8).  They "went everywhere preaching the word" (Acts 8:4).  Paul later declared that the gospel had been preached "to every creature under heaven (Colossians 1:23).
 
         The Kingdom and the Church Both Came
 
Jesus told the apostles, "I will build my church... And I will give unto thee the keys of the kingdom of heaven" (Matthew 16:18-19).  Jesus clearly used "church" and "kingdom" to refer to the same institution.  He gave the keys to what He built.  The "house of God" (the church) (1 Timothy 3:15) had been established, and the gospel was to be preached to all the world.
 
         Christ as King Over His Kingdom
 
Paul wrote in 1 Timothy 6:15, speaking of Jesus, "He who is the blessed and only Sovereign, the King of kings, and Lord of lords."  Since God has "delivered us from the power of darkness, and has translated us into the kingdom of his dear Son (Colossians 1:13), we must live as good citizens of His kingdom.  As our beloved King, He is worthy of obedience and loyalty.  We are subjects of the King of glory, knowing that He is reigning now, and:  "He must reign, till he has put all enemies under his feet.  The last enemy to be destroyed is death" (1 Corinthians 15:25-26).