No one can lay God and the Kingdom on the table before another man; even the believer cannot do it for himself. But however strongly unbelief may feel itself thereby justified it cannot forget the eerie feeling induced by the words “Yet perhaps it is true.” That perhaps is the unavoidable temptation which it cannot elude, the temptation in which it, too, in the very act of rejection, has to experience the unrejectability of belief. In other words, both the believer and the unbeliever share, each in his own way, doubt and belief, if they do not hide away from themselves and the truth of their being. Neither can quite escape doubt and belief; for the one, faith is present against doubt; for the other through doubt and in the form of doubt. It is the basic pattern of man’s destiny only to be allowed to find the finality of his existence in this unceasing rivalry between doubt and belief, temptation and uncertainty. Perhaps in precisely this way doubt, which saves both sides from being shut up in their own worlds, could become the avenue of communication. It prevents both from enjoying complete self-satisfaction; it opens up the believer to the doubter and the doubter to the believer; for one it is his share in the fate of the unbeliever, for the other the form in which belief remains nevertheless a challenge to him.
– Joseph Cardinal Ratzinger, Introduction to Christianity