Basically the nieghbouring kingdom lost their seintess, due to an influx of monsters. So they were looking desperately to get a new one. Just so happens that the stars aligned (and put together the dumbest people on the planet *khm* I mean) and their neighbours (where MC is) decided to sell the most powerful person on the planet (parents didnt like her as much as their other daughter that was also a saintess (explained why in the story), and the prince was scared of her, wanted to marry her sister instead, also she was making medicine for the king, which the prince also wanted to get rid of).

The prince tells her shes dull becauses shes perfect (because she was trained by her master very harshly during childhood to deal with anything that comes at her (due to masters plot twist reasons)), and she just naturally doesnt express much emotion. So this is the setup and then parts of the story all evolve from this:

First part: After the sell, the early story is really carried by the saintess' sister. She actually figures sh*t out on her own and steps up to take revenge for the sister while saving the kingdom from the prince (well his idiotic decision making during the upcoming crisis, like selling an op saintess). Meanwhile the "dull" MC opens up in the neighbouring kingdom that treated her warmly and with respect (as it should be shes basically the most op person in their world, and also really diligent and kind).

Second part: The influx of monsters gets worse, and the 400 year crisis decends, so the MC does some character growth (personality not power wise) and with the help of others resolves it, this is still related to the title since the previous saintess died due to the influx of monsters due to the approaching crisis

Third part: The main bad guy shows up (the demon duke mentioned earlier in the story) and posses the prince she was engaged to, so we deal with that, this is still mostly related to the second part and so the title and its circumstances

Fourth part: The pope dies, and she is named as the next pope, but she suspects somethings wrong since nothing like this happened before and due to new circumstances she just really doesnt want the position, also the one passing on the will is the brother of the deceased saint. So this is still related to the circumstances of her being sold.

They were like if a god wakes up they are very angry and try to kill people. Thats why we have this staff, if they wake up it puts the to sleep again. In like 10 chapters bad guy wakes up a god, knocks some people out, the MC uses the staff and he went back to sleep (this was like 50% of 1 chapter) I was like... huh, well I guess that happened, so ye like I said really straight forward as far as the boss fights go, they might go into more detail in LN though not sure. I was disappointed that they didnt explain more about the previous saintess (it just said she was sickly and ended up dying, I assumed it was due to the influx of monsters and she overworked herself but the real reason was never stated, although it was mentioned multiple times that she tried her best to make the country better and protect it, so her dying to the influx of monsters while having a bad constituion is not a big leap in my opinion) and her dad, but considering LN 4 synopsis mentions to go a bit more into dad lore, the saintess might get more of a backstory too.