General:
* Faith: This emphasizes the belief system aspect of religion.
* Belief system: This is a more neutral term, simply referring to a set of beliefs.
* Spirituality: This focuses on the personal connection with something greater than oneself.
* Cult: This implies a smaller, often controversial, group with a strong leader.
* Denomination: This refers to a specific group within a larger religion.
Specific aspects:
* Dogma: This emphasizes the set of beliefs and principles.
* Worship: This focuses on the practices and rituals.
* Theology: This refers to the study and understanding of religious beliefs.
* Doctrine: This refers to a specific set of teachings.
The best synonym will depend on the specific meaning you want to convey.