Theology
Defaults: IQ-6 or Religious Ritual (same)-4.
This is the study of a particular religion: its gods, cosmology, doctrines, scriptures, etc. You must specialize by religion. There are usually no defaults between specialties, but the GM might permit a default at -4 or so for belief systems that have similar origins, or where one is derived from the other.
Alternatively, you may study the similarities and differences between religions; this is Theology (Comparative). The Theology of any religion routinely studied by scholars in your game world defaults to this specialty at -5.
To be a priest or holy man at TL1+, you must have both Theology and Religious Ritual skill for your religion. TL0 shamans need only learn Religious Ritual.
You do not necessarily believe in the religion you study – faith comes from within, not from book learning! If you do, you may ask the GM to make a secret roll against your Theology skill when confronted with moral uncertainty. On a success, the GM will advise you on which course of action "feels" right, given your beliefs and understanding of scripture.