She wants to become the leader of church to stop the persecution, but her life as a street rat also made her desire power in general.
She hopes to do this by proving that she is the strongest saintess.
She is magically the strongest of the three but becouse of her sheltered upbringing and lack of experience makes her appear more spoiled and selfish than she actually is.
Don´t get me wrong, she IS a spoiled brat because the church half worshiping her potential as a powerfull saintess, but she is not nearly as stupid and small minded as she appears.
She is naive, inexpereienced and overconfided but she is not stupid