Great Cycle of Being
From Diablo Wiki
The Great Cycle of Being is the main ideology in the Priests of Rathma's (or Necromancers, as they are also called) doctrine. It says that every living being goes through a cycle of life and death which is unavoidable.
Belief in this cycle seems to give a Necromancer great fortitude and he/she does not fear death as much as others, accepting it a part of the cycle.
This ideology is not evil or good, it's about balance between order and chaos. When Diablo and his demonic minions tried to invade Sanctuary, the priests reacted strongly, and did all in their power to stop him and his brothers.