The Religion skill encompasses knowledge about gods, sacred writings, religious ceremonies, holy symbols, and theology. This knowledge extends to information about the undead and about the Astral Sea, including the creatures of that plane.
Training in this skill represents academic study, either formalized or as a hobby. Those that have training in the skill are likely to know esoteric information in the field of study.
Make a Religion check to recall a piece of relevant religious lore or to recognize a religion-related clue (see “Knowledge Checks”).
Immortal and Undead
Make a Religion check to identify a creature that has the immortal origin or the undead keyword (see “Monster Knowledge Checks”).
Craft a simple nonmagical holy symbol or other sacred object (moderate DC)
Preside over a known religious ceremony (moderate DC)
Soothe grief-stricken or panicked peasants by chanting a hymn (hard DC)
Published in Player's Handbook, page(s) 187, Rules Compendium, page(s) 151.