There are two kinds of people in the world: those who say that there are two kinds of people in the world and those who don't.
I'm one of the first.
I say, there are those who believe that we can create Heaven on Earth and those who believe that, although we can improve this world (and dramatically!), God's Kingdom is not of this world.
Those who believe that they have a blueprint for Heaven on Earth in fact have a blueprint for Hell. And they have been successful in bringing their plan to fruition. Hitler, Stalin, Pol Pot, Mao, Kim Jong Il, Castro and their like have created Hell precisely because they had the plans for Heaven in their pockets.
I love this paragraph from C S Lewis, quoted by the magnificent James Delingpole in Watermelons:
It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated; but those who torment us for our own good will torment us without end, for they do so with the approval of their own conscience.