Ibn Tufayl's Hayy ibn Yaqzan (in translation).
This is a philosophical novel written in the 12th century about a feral child who comes to understand God, and, eventually the pitfalls of blind faith, while acknowledging the roles it plays for the masses. Ibn Tufayl was an Andalusian philosopher, theologian, physician, and astronomer who also served as vizier.
It includes the first autopsy described in a novel and is said to have been the inspiration for Robinson Crusoe. It also was influential on European philosophers of the Enlightenment.
http://www.amazon.com/Ibn-Tufayls-Hayy-Yaqzan-Philosophical/dp/0226303101