Religion's checks are based on the Intelligence ability score.
You have picked up knowledge about gods, religious traditions and ceremonies, divine effects, holy symbols, and theology. This knowledge extends to information about the undead and the Astral Sea, including the creatures of that plane.
If you have selected this skill as a trained skill, your knowledge represents academic study, either formalized or as a hobby, and you have a better chance of knowing esoteric information in this field.
Make a Religion check to remember a useful bit of religious knowledge or to recognize a religion-related clue. See “Knowledge Checks”.
Immortal or Undead
Make a Religion check to identify a creature that has the immortal origin (a creature of the Astral Sea) or the undead keyword. See “Monster Knowledge Checks”.