People hating on a girl literally named Mary Sue (sic!) are making my sides hurt from laughing. Hello, did you all lost your prescriptions or something? This girl was literally chosen by gods. Three gods, to be precise. The sick irony is that, all three of them merely chose a semi-random person to receive a blessing in order to remind people of deities' existence since the level of faith has lowered in this world. But they independently from each other chose the same girl. Check Mary Sue trope, if you somehow are unaware of it.
This child is naive and simple, yes, but you are really letting your personal opinions screw your perception of the story. She will be religious, she was made that way by higher forces. She acts exactly as she is supposed to inside this setting.
I have only one explanation as to why some people fail to understand which was both shown and said to us: people don't read and don't remember stuff. Tsk-tsk, it's not author's fault that you don't read the provided material and then can't comprehend something. How unbecoming.