In a fantasy setting, where magic is abundant, a commonly seen rule is that no one can master multiple or opposing schools of magic-- One can either learn divine or unholy magic, but not both. The reasons for this may vary. It may be that the teachers of either school don't particularly get along very well, it may be that the talent for any given type of magic is an inherited trait, or it may simply be that all you used to learn one type contradicts what's used to learn the other, and most people just can't get over that hump. In some settings, this extends to all forms of Functional Magic: Hermetic Magic, Psychic Powers, Ki-powered Supernatural Martial Arts, etcetera.