I believe it is because there are so many false doctrines that arise out of the belief that the kingdom has not yet be established. The most prominent of these is the doctrine of premillennialism.
There is general agreement that the Lord's church was established on the first Pentecost after Christ ascended into heaven. Consequently, if one can prove that the kingdom is the church and that the church was established on Pentecost, we have the answer to our question.
To prove that the church and the kingdom are the same thing, we need only to look at Matthew 16. In verse 18, Jesus tells Peter that He will build his "church" on the rock of fact that He is the Son of God. In the very next verse (19) Jesus tells Peter the He will give him the keys of the "Kingdom." It would be very presumptuous of any one to say that Jesus is talking of two different institutions here. The context demands that "church" and "kingdom" are both referring to the same thing.
Also, if the kingdom is not in existence today, there must be some very old people living somewhere in the world. In Mark 9:1, Jesus said that some that stood before him would not die before the kingdom had come with power. That statement was made nearly 2000 years ago.
We can prove that the kingdom is in existence today by the fact that it is referred to in the Bible as being in existence. Colossians 1:13 tells us that we have been translated into the Kingdom of God's dear Son. It has to exist for that to be possible. Hebrews 12:28 tells us that we have received a kingdom which cannot be moved. Also, the apostle John said that he was in the Kingdom (Revelation 1:9).
-- Ralph Price --