The Christian optimism is based on the fact that we do not fit in to the world.

They cannot indulge in any detailed or merely logical defense of life; that would delay the enjoyment of it. These higher optimists, of whom Dickens was one, do not approve of the universe; they do not even admire the universe; they fall in love with it. They embrace life too close to criticize or even to see it. Existence to such men has the wild beauty of a woman, and those love her with most intensity who love her with least cause.

We do not admire, we hardly excuse, the fanatic who wrecks this world for love of the other but what are we to say of the fanatic who wrecks this world out of hatred for the other? He sacrifices the very existence of humanity's to the idleness of the altar and the emptiness of the throne. He is ready to ruin even that primary ethic by which all things live, for his strange and eternal vengeance upon someone who never lived at all.

I still think sincere pessimism the unpardonable sin.