Faith as ultimate concern is an act of the total personality. It happens in the center of the personal life and includes all its elements. Faith is the most centered act of the human mind. It is not a movement of a special section or a special function of man's total being. They all are united in the act of faith.
Paul TillichOut of the element of participation follows the certainty of faith; out of the element of separation follows the doubt in faith. And each is essential for the nature of faith. Sometimes certainty conquers doubt, but it cannot eliminate doubt. The conquered of today may become the conqueror of tomorrow. Sometimes doubt conquers faith, but it still contains faith. Otherwise it would be indifference.
Paul TillichIn the courageous standing of uncertainty, faith shows most visibly its dynamic character.
Paul Tillich