Life is doubt, and faith without doubt is nothing but death.

Miguel de Unamuno (1864-1936) Spanish philosopher and writer [Miguel de Unamuno y Jugo]

“Poesias” (1907)

Alt trans. "Faith which has not doubt is dead faith." Alt trans. "Faith which does not doubt is dead faith." (Cited as La Agonía del Cristianismo [The Agony of Christianity])