Entoverse by James P. Hogan (1992, Paperback, Reprint)

Meanwhile, in a very different universe, where magic worked and nothing physical was predictable, holy men caught glimpses of another place, a place where the shape of objects remained unchanged by motion, and cause led directly and logically to effect. And the best part was that when the heart was pure, the mind was focused, and circumstances were right, some lucky souls could actually make the transition to that other universe. If only they all could...