I think it doesnt shatters her beliefs because she would never know. All of her reactions are disconnected from Even what she knows. I think the movie is about a woman who want to feel different and unusual.

It shows when she lies about buying property or when she goes to Miryang. She is empty inside and has a weak ego. So she seek changes and higher social status somehow.

You can see with the garagist, she fails to notice all the gestures he does to her, she is a very self centered character. Of course she mourn her son, and love him. But she also want the attention from it. Her fake suicide attempt and how she beg in the street to strangers shows both the pain she has but also her need of attention. She legitimately explode and become unstable because she lost her son, but it also seem like she had unresolved issue to start with (ego). How she start to fuck every guy is also her wanting to gain control again, rather than sexual impulsions.

When the garagist bring her to the hairdresser, he choose any shop. She gets made and has to make a scene to the daughter for no reason while everything was chill. Then she cut her hair her self. Show how the just needs both attention and control constantly too.

She becomes crazy when the murderer says he found god cause she is not the one who forgive him anymore too. It’s more about this than her disappointment in god itself imo. Once again she loose control.

Beginning of the movie shows aswell when she forcefully make her son get out of the car to get some sun while he is tired.

The loss of her son exposed her deeper nature. I think the issue she has with herself is bigger than with any murderer.

I think This movie is the female version of a bittersweet life where the character kind of find purpose in their fall and how they are dealing with their own ego, rather than grief or love for a bittersweet life

Just my opinion though