The existence of the Remnant, even among believers in the twelve, is highly and sometimes hotly debated. Its nature, likewise, is also highly and sometimes hotly debated.
Among those who believe, some say it is a sentient entity and belongs with the rest to whom they consequently refer as “the thirteen.” Others say it is merely residual energy from the primal scream that shattered the one who walked alone.
Some wizards view spellcraft as the science of channeling energy from the Remnant. Some view any talk of the one who walked alone, the twelve, and the Remnant as hogwash. To them, spellcraft is the art of channeling various energies that are naturally occurring: the energies of life, the energies of the suns, the energies of the planets and the moons, the energies of the lands and the seas, and the energies of the elements, and energies from other planes. The twelve, if they do indeed exist, are as any other extra planar being or are merely highly advanced wizards.
These wizards believe that clerics and druids have somehow found a more intuitive way of tapping into these energies through some sort of meditation. As a group, they are both fascinated and envious. Their efforts to learn from the priests have often ended up in frustration due to irrational scroll thumping. Less frequently, the priests have successfully converted a wizard.
This is not to say that converted wizards must give up wizardry. A converted wizard may become a believer and remain a wizard. Or they may become a follower and give up wizardry for priesthood. Or at the DM’s discretion, they may split their class (which will require them to advance as a priest exclusively until the class levels are equal).
Priests view the Remnant as a myth. They view wizards as being misguided and unwittingly seduced by one of the twelve who is likely of an opposing alignment to the wizard. As a priest can never be certain which of the twelve is exploiting the wizard, they tend to be wary of wizards unless they can convert them. After all, there are all those stories of good wizards who have turned evil (or, for that matter, evil wizards who have turned good).