The trouble with kingdoms of heaven on earth is that they're liable to come to pass, and then their fraudulence is apparent for all to see. We need a kingdom of heaven in Heaven, if only because it can't be realised.
One of the peculiar sins of the twentieth century which we've developed to a very high level is the sin of credulity. It has been said that when human beings stop believing in God they believe in nothing. The truth is much worse: they believe in anything.