Paul Tillich said this quote

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

~ Paul Tillich

Sometimes I think it is my mission to bring faith to the faithless, and doubt to the faithful.

~ Paul Tillich

Faith as the state of being ultimately concerned implies love, namely, the desire and urge toward the reunion of the seperated.

~ Paul Tillich

Faith includes both an immediate awareness of something unconditional and the courage to take the risk of uncertainty upon itself. Faith says Yes in spite of the anxiety of No.Biblical Religion and the Search for Ultimate Reality

~ Paul Tillich