Just at 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 be 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.
--Cardinal Josef Ratzinger (Pope Benedict XVI), Introduction to Christianity, 1968
