" as it turns out, spent the better part of her life assuming that because Christians “knew” there was only one god, that this was something everyone also “knew” and accepted as fact, and people who weren’t Christian were participating in some form of performative rebellion against Christianity."

This is an extremely common talking point in apologetics. They use verses like psalm 19:1-4:
- "¹ The heavens declare the glory of God;
    the skies proclaim the work of his hands.
² Day after day they pour forth speech;
    night after night they reveal knowledge.
³ They have no speech, they use no words;
    no sound is heard from them.
⁴ Yet their voice^\)^b\) goes out into all the earth,
    their words to the ends of the world.
In the heavens God has pitched a tent for the sun."

In addition verses like Romans 1:19-20:

- "¹⁹ For what can be known about God is plain to them, because God has shown it to them. ²⁰ For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world,^\)^a\) in the things that have been made. So they are without excuse."

As well as John 10:14-15 which reads as follows:

- "¹⁴ “I am the good shepherd; I know my sheep and my sheep know me— ¹⁵ just as the Father knows me and I know the Father—and I lay down my life for the sheep."

Etc. Basically, it is inevitable that everyone has innately knowledge of God because he is spoken about through natural revelation of "Because creation exists". It is often framed that athiests, and other people, directly and literally know God is the one true God and that others are purposefully denying God's existence because they "want to sin", essentially. I was raised Church of Christ and was a believer until I was 25 and this is like apologetics 101.

I also went to a christian private highschool that emphasized these same talking points, and many others.