When we exhort people to Faith as a virtue, to the settled intention of continuing to believe certain things, we are not exhorting them to fight against reason. The intention of continuing to believe is required because, though Reason is divine, human reasoners are not. When once passion takes part in the game, the human reason, unassisted by Grace, has about as much chance of retaining its hold on truths already gained as a snowflake has of retaining its consistency in the mouth of a blast furnace.

C.S. Lewis, "Religion: Reality or Substitute?", in "Christian Reflections".