Miguel De Unamuno said this quote

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

~ Miguel De Unamuno

Those who believe that they believe in God, but without passion in their hearts, without anguish in mind, without uncertainty, without doubt, without an element of despair even in their consolation, believe only in the God idea, not God Himself.

~ Miguel De Unamuno

Faith which does not doubt is dead faith. -Miguel de Unamuno, philosopher and writer (1864-1936)

~ Miguel De Unamuno

Yes, yes, I see it all! — an enormous social activity, a mighty civilization, a profuseness of science, of art, of industry, of morality, and afterwords, when we have filled the world with industrial marvels, with great factories, with roads, museums and libraries, we shall fall exhausted at the foot of it all, and it will subsist — for whom? Was man made for science or was science made for man?

~ Miguel De Unamuno