The church did not come into being until Pentecost. So when Jesus spoke the words quoted in Luke 22:18 Pentecost was still in the future.
Luke 22:18 was quoted to show that the Kingdom is not heaven. It was not quoted to show the Kingdom is not the Church.
As for Luke 21:27-31: "‘27 And then they will see the Son of man coming in a cloud with power and great glory. 28 Now when these things begin to take place, look up and raise your heads, because your redemption is drawing near.’
29 And he told them a parable: “Look at the fig tree, and all the trees; 30 as soon as they come out in leaf, you see for yourselves and know that the summer is already near. 31 So also, when you see these things taking place, you know that the kingdom of God is near.” [Luke 21:27-31]
You will get a totally different perspective if you include the next verse:
“32 Truly, I say to you, this generation will not pass away till all has taken place.”
[Luke 21:32]
That event occurred in that very first century generation with the birth of the church on Pentecost. The Kingdom has been here for almost 2,000 years now. In Mark 9:1 we read where Jesus tells the multitude:
"1 And he said to them, “Truly, I say to you, there are some standing here who will not taste death before they see that the kingdom of God has come with power.” [Mark 9:1]
Surely, Luke 21:27 “And then they will see the Son of man coming in a cloud with power and great glory” could only be speaking about the second coming of Christ. The Book of Revelation says the same thing about the second coming: “Behold, he cometh with clouds, and every eye shall see him…” (Rev. 1:7).
When the Church was founded on the day of Pentecost, did anyone see Christ coming in a cloud with power and great glory? But “EVERY EYE” was supposed to see Christ coming!
If the Church is the Kingdom of God why were the apostles still preaching about “the things concerning the kingdom of God” years after the founding of the Church? – Acts 8:12, 19:8, 20:25, 28:23, 28:31.
When one is “in the faith” that person is in God’s Church. If the Church is the kingdom, how come those already in the Church must continue “in the faith” to enter the kingdom? “Confirming the souls of the disciples, and exhorting them to continue in the faith, and that we must through much tribulation enter into the kingdom of God” (Acts 14:22).
If the Church is the Kingdom of God, why did Paul say that “flesh and blood cannot inherit the kingdom of God”? The churches of this world, the Catholic Church included, have all flesh and blood human beings as its members. It just could not be the Kingdom of God.
“This generation” could mean the Jewish RACE and not necessarily the generation of the Jews then living in Jesus’ time.
Mark 9:1 was fulfilled six days after Jesus made the statement. The very next verse tells us that, “And after six days Jesus taketh with him Peter, and James, and John, and leadeth them up into an high mountain apart by themselves: and he was transfigured before them.”
Jesus showed His disciples [in a vision] a fleeting glimpse of the power of the Kingdom – how the Kingdom could make Elijah and Moses, who were both dead and buried, be with Jesus, alive and in a glorified condition.
“And his raiment became shining, exceeding white as snow; so as no fuller on earth can white them. And there appeared unto them Elias with Moses: and they were talking with Jesus.” (v. 3-4).
It was the power of God’s Kingdom to resurrect Moses and Elijah that really got the attention of Christ’s disciples: “… they kept that saying with themselves, questioning one with another what the rising from the dead should mean.”
As for heaven that too is part of the church. After all the church is the Body of Christ and the saints in heaven are certainly in the Body of Christ.
No saint is in heaven. As Christ said, “NO MAN hath ascended up to heaven, but he that came down from heaven, even the Son of man” (John 3:13).
Heaven IS NOT the Church and is also NOT the Kingdom of God. Heaven existed eons and eons ago; the Church was founded only recently; while the Kingdom is yet to come. “Our Father, WHO ART IN HEAVEN, hallowed be thy name, THY KINGDOM COME…”
Christ will eventually deliver the Kingdom of God to the Father: “Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power” (I Cor. 15:24). But the Father is already in heaven for, “I say unto you, Swear not at all; neither by heaven; FOR IT IS GOD’s THRONE” (Mat. 5:34).