The view that the Kingdom is “not yet” and will come when Jesus returns in power is the last residue of dispensationalism. The truth is that the kingdom described in the New Testament cannot be established by his returning in power, because there are only two ways that he could do it.

1. Human nature could be changed so that we lose the freedom to sin, but it would become a kingdom of automatons.
2. Jesus could use force to coerce unwilling men to accept his authority, but it would cease to be the voluntary kingdom.

In either case, the resulting kingdom would be much diminished from the Kingdom described in the New Testament.

The only way for this kingdom to come is for people to freely choose to accept Jesus authority. That can only happen through the Holy Spirit renewing human hearts. God has set things up so that can happen now. Jesus had to go away so that could happen.

The widespread view that this cannot happen, is actually a belief (unbelief) that the Holy Spirit can not do it, but Jesus can. This is a distorted form of Trinitarianism, where there is one God in two and half persons.