From the Back of the Book:
On a world with no gods, what do priests serve? The priest class on Athas is unique and distinct from the priest classes of any other AD&D campaign world. Athasian priests worship no gods, collect no followers, and carry edged weapons.
Earth, Air, Fire, and Water answers all your questions about this singular class of clerics. Here are the elemental priests who strive to return their barren and battered world to its former state of lush green valleys, forested mountains, and cool waterfalls.
Here are the paraelemental priests who oppose the restoration of Athas, serving the paraelements whose power and might is growing as defiler magic continues to plunge Athas further into decay and ruin.
New priest kits allow PCs to become an Athasian elemental priest of Earth, of Air, of Fire, or of Water--but they can only serve one, for the elements are jealous masters. Each kit contains unique powers, limitations, and proficiencies. Each offers four different ways in which to bring about the regeneration of Ahtas: as a Wanderer, as a Guardian of a shrine, as a City Cleric, or as a Shaman of a tribe. But beware the paraelemental priests--their success depends on the failure of the elemental clerics!