Both Uses

heathen
in
To All the Boys I've Loved Before
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Mrs. Kavinsky's smiling now She puts another piece of pizza on Owen's plate and says, "You're heathens.†
p. 211.3

heathens = people who are not civilized or not moral -- typically said humorously OR an offensive term for people who do not believe in a preferred religion
"Well ...Kitty might be part heathen," I admit.†
p. 211.4 *

heathen = someone who is not civilized or not moral OR an offensive term for a person who does not believe in a preferred religion

Definitions:

(1) (heathen)
an often offensive (sometimes humorous) word for someone seen as uncivilized or immoral, especially because she does not share the speaker’s religion or comes from a culture unfamiliar with it

This word is usually judgmental or insulting, so today it is used mostly in joking contexts or when describing older attitudes.
(2) (meaning too rare to warrant focus)
More rarely, heathen can refer to a person who lacks culture or good taste.