You are using the word "christian" as an adjective, not a noun; therefore you are not referring directly to people who believe in Jesus, only the stereotypical image that has been bestowed upon them by others.
It's absolutely a reference to Christians to say to someone "you're so
Christian." Christians are the stem of the metaphor, and very obviously have something to do with it.
"He's a bit of a flat earther when it comes to global warming" is not inviting you to check with the FES about their views on global warming.
Calling someone a Flat Earther is making a direct comparison to us, members of the Flat Earth Society, who have for over 150 years believed in a Flat Earth contrary to popular opinion and media hype. We're the stem of the comparison, quite obviously, because you're calling someone a Flat Earther as a metaphor.
It was argued that "Flat Earther" had nothing to do with the FES. This is incorrect. We are responsible for the very root of that phrase, not some unknown Flat Earth group who has held contrarian beliefs in the face of adversity.
The phrase "Flat Earther" isn't in reference to the Ancient Egyptians and other ancient societies who believed in a Flat Earth in times of acceptance. The metaphor is being used to describe someone who holds beliefs which the public no longer holds true. We're the only group of FE'ers who have believed in an FE in modern times. The metaphor is indisputably referencing us.