In order to match "Αυτή" isn't it "θρησκευόμενη" (rather than "θρήσκα")?
These two words are synonyms. Θρήσκος becomes θρήσκα and θρησκευόμενος becomes θρησκευόμενη.
Is there any way to tell whether an adjective ending in -ος in the masculine will form its feminine in -α rather than -η ?
Please read kirakrakra's comment. here. Add the adjective "μεγάλος" to that list ;)