I was very involved in my campus religious group while in college, and my minister wrote and LOR for me. She knows me very, very well, and had seen me work (I was an officer in the group for 3 years, president for 1, and organized a mission work trip for 30 people). She wasn't just a character reference for me, she was a work reference.
If your gf's faith and activities are important to her, then she should include it in her app and in her LORs. Just make sure she's not offensive in what she says-- for instance the sentence "It is one of my missions in life to convert everyone to worshipping the God of Cheese and to wash away all non-believers in an avalanche of velveeta," probably has no business being in her PS. However, a sentence like "My experiences in the Cheese Faith has enriched my life and given me opportunities to give to the community by working in soup kitchens by giving cheese to the hungry," would be a better choice.
(yes i know i'm being silly, but that's my point...you don't know who is reading the PS, so keep it neutral)