I might be confused from something I saw on another excersize, but does the suffix for instrumental reflect the sex of the subject? On jest atiestem, ona jest atiestą? Clearly that's not the case here, but I recall seeing that with Brytyjką/Brytyjkiem, or something like that.
Brytyjką/Brytyjczykiem (female/male), ateistką/ateistą (female/male).
It's not that Instrumental reflexts the sex, it's just that most of such nouns reflext the sex and of course it's seen in Instrumental as well.
how come you can say "he's Christian" without the article but not "he's atheist"?
also there's a weird bug on the site that if a word starts with a you can omit the article and it just marks it as a typo
Interesting question! To me (native Brit. Eng. speaker), "Christian" sounds like it could be an adjective or a noun, so with or without an article both sound fine. "Atheist" feels more like a noun (only), so it would need an article. If I wanted to omit the article, I'd feel I'd have to make the word into an adjective ("Atheistic"?) to make it work.
It's hard to make a rule about this; it just feels right to me that way.