The kingdom has come, repent and believe! Heaven will be a place where we go after we die.
It started with that, and then slowly the theological teaching has changed. We can help bring God's kingdom on earth by living the biblical way, loving one another, etc. But the ultimate heaven will be much better than the current state of heaven.
Then something struck me. If heaven can be a place on earth... Won't hell be a place on earth as well? We talk so much about hell being filled with fire and gnashing teeth. That pretty much sums up some parts of the world where God isn't around. But the ultimate hell will be much worse than the current state of hell.
I now see the parallel. Do you?