As I understand it, the Holy Church teaches that there are two worlds: the world of everything we can see and hear and touch, and another world, the spiritual world of heaven and hell. Barnard and Stokes were two — how shall I put it — renegade theologians who postulated the existence of numerous other worlds like this one, neither heaven nor hell, but material and sinful. They are there, close by, but invisible and unreachable.

The Master of Jordan College to the Librarian, in Ch. 2 : The Idea of North - His Dark Materials - The Golden Compass (1995)