Doubt isn't the opposite of faith; it is an element of faith.

Tillich’s theology treats faith not as certainty about propositions but as an existential “ultimate concern” lived under conditions of finitude and ambiguity. On that view, doubt is not a defect that cancels faith; it is the inevitable companion of any serious commitment made without absolute, coercive proof. The quote underscores a mature, non-triumphalist spirituality: genuine faith can include questioning, anxiety, and intellectual struggle, because it is directed toward what ultimately matters rather than toward airtight certainty. It also challenges simplistic oppositions—faith vs. doubt—by suggesting the real opposite of faith is not doubt but indifference or disengagement from ultimate meaning.