"Faith which does not doubt is dead faith."
Miguel de Unamuno y Jugo (1864-1936)
A pre-existentialist philosopher and writer (see also Soren Kierkegaard). He wrote a lot about the inconsistencies of faith and reason, religion and freedom of thought.
He was very torn in his own religious beliefs. He wrote a lot about death and seems kind of obsessed with it, or rather with the fact that reason for him had replaced religion, but offered no substitute comfort regarding death. He badly wanted to believe in God, but based on reason he just couldn't do it.
He could speak and write in fourteen languages, but most of his books were written in Spanish and his mother tongue was Basque.
His two most interesting books (IMHO) are Mist (1914) and Saint Manuel Bueno, Martyr (1931).
In Mist, Unamuno creates a bunch of characters and casts himself (the author) in the role of God. One of the characters, Augusto Perez, meets the author (Unamuno/God), realizes that he is a fictive character, and commits suicide.
In Saint Manuel Bueno, a country priest who is secretly an athiest nevertheless converts and ministers to his parishoners, his logic being that even though he knows God doesn't exist, the belief that He does is the best comfort he can offer poor people with little other hope.