Both the believer and the unbeliever share, each in his own way, doubt and belief, if they do not hide from themselves and from the truth of their being. Neither can quite escape either doubt or 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 certainty. 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.
Josef Ratzinger, Introduction to Christianity (2nd edition)