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That's because in the story that she knows, the high priest is a good person, and that as the prophesied demon lord, she is the bad one. Up until now the fmc is only focused at proving that she will not become the demon lord that she should be like in the original story. She doesn't have any reason to doubt the high priest's character yet because she hasn't really stopped associating everything to the story she has seen in her past life. She isn't suspicious of the high priest just yet because she doesn't know who's actually the one manipulating everything behind. We as the readers, have an access to what each characters are doing, thinking, and plotting. But the characters themselves can only make guesses on their own or make assumptions, if they actually were able to guess correctly what the other characters will be doing next, that would be so damn boring.. I'd hate to have a character wherein they would always be right about what will happen next