All 3 Uses

heathen
in
Everything, Everything
(Edited)

Madeline: You're not a heathen.
p. 52.1

heathen = someone who is not civilized or doesn't believe in the right religion
Madeline: You are a heathen.
p. 52.1
Madeline: You've been downgraded from heathen to heretic.
p. 52.4 *

heathen = someone who doesn't believe in the right 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.