If doubt appears, it should not be considered as a negation of faith, but as an element which was always and will always be present in the act of faith. Existential doubt and faith are poles of the same reality, the state of ultimate concern…But serious doubt is confirmation of faith. It indicates the seriousness of the concern, its unconditional character. This also refers to those who as future or present ministers of a church experience not only scientific doubt about doctrinal statements–this is necessary and perpetual as theology is a perpetual need–but also existential doubt about the message of their church, e.g., that Jesus can be called the Christ.
Paul Tillich, The Dynamics of Faith, 22.