Just as we have already recognized that the believer does not live immune to doubt but is always threatened by the plunge into the void, so now we can discern the entangled nature of human destinies and say that the no believer does not lead a sealed-off self-sufficient life, either. However vigorously he may assert that he is a pure positivist who has long left behind him supernatural temptations and weaknesses and now accepts only what is immediately certain, he will never be free of the secret uncertainty about whether positivism really has the last word. Just as the believer is choked by the salt water of doubt constantly washed into his mouth by the ocean of uncertainty, so the non- believer is troubled by doubts about his unbelief, about the real totality of the world he has made up his mind to explain as a self-contained whole. He can never be absolutely certain of the autonomy of what he has seen and interpreted as a whole; he remains threatened by the question of whether belief is not after all the reality it claims to be. Just as the believer knows himself to be constantly threatened by unbelief, which he must experience as a continual temptation, so for the unbeliever faith remains a temptation and a threat to his apparently permanently closed world. In short, there is no escape from the dilemma of being a man. Anyone who makes up his mind to evade the uncertainty of belief will have to experience the uncertainty of unbelief, which can never finally eliminate for certain the possibility that belief may after all be truth. It is not until belief is rejected that its unpredictability becomes evident.
--Joseph Ratzinger (Pope Benedict XVI),
Introduction to Christianity, 1969
