So Cybel has never shown anything like the Saintess's power before, however someone did determine she was the saint despite this. The Step-mother said she saw Annika use holy power, that no one else saw, and told the prince. The prince just believes her and declares that Cybel is the fake saint. It's not until months later that he becomes suspicious about whether Cybel was actually a fake saint. So he just never conferred with any church member, or the person that declared Cybel was the saint in the first place, about if Annika had holy power?
This is a pretty significant plot hole cause it starts the story and I just assumed the step-mother paid off a priest. Apparently the prince just never bothered to ask anyone else at all.