The believer can only perfect his faith on the ocean of nihilism, temptation and doubt; he has been assigned the ocean of uncertainty as the only possible site for his faith. On the other hand the believer is not to be understood undialectically as a mere man without faith.

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 even the non-believer does not represent a rounded and closed existence. However vigorously he may assert that he has long left behind him supernatural temptations and weaknesses and now accepts only what is immediately certain he will never be free of secret uncertainty.

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 which 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 whether the belief is not after all the reality which 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 so a threat to his permanently closed world.

            - Joseph Ratzinger, Intro to Christianity