Mentally unstable or just religious?

Exactly.

She was mentally unstable. Religion was just her vehicle. Being religious does not equate to mental instability.

If I understood correctly, she had just recently turned to religion. And this was after whatever event got her fired from her prior job. She's mentally unstable. I guess you could say her mental instability may have caused her to turn to religion.

Brain tumor imo: https://moviechat.org/tt7557108/Saint-Maud/6031eeba5b186a48643b6541/so-what-was-she-spolier?reply=6072caf0fbc117538f6dafbd