This question may be tongue-in-cheek, and some of the answers may be, too. But if anyone is serious:

The reason has nothing to do with favoritism of Christians in particular. It's simply because — in English, specifically — the names of religions and their adherents are PROPER NOUNS, and proper nouns are always capitalized:

Christianity/Christians
Judaism/Jews
Islam/Muslims
Hinduism/Hindus
Buddhism/Buddhists
Wicca/Wiccans
➥...even Pastafarianism/Pastafarian.

Proper names are the words that define specific entities.

Words like "atheism/atheist," "agnosticism/agnostic," etc. are are regular nouns, and therefore NOT capitalized.

To demonstrate that this HONESTLY isn't favoritism, here are some other words that are NOT capitalized:

apostles
baptism
born again
church
ecumenical

In English, the term "God" is capitalized when referencing the Judeo-Christian or Abrahamic or monotheistic God (in Judaism, Christianity and Islam mostly), and lower-cased when referring to other gods, or polytheistic gods, or gods in general.

Again, the reason isn't favoritism, so much as simplification and shorthand. To make understanding easier. That's all.

A few points:

✔ This is a rule in English. Not all languages capitalize proper nouns, or even have them.

✔ Grammatical rules in English didn't start getting organized before the 18th Century. Before that, words were capitalized, or not, willy-nilly.

✔ Nobody is thrown in jail for not following grammatical rules. You don't HAVE to capitalize "Christian" if you don't want to. But the reason rules exist in English is shorthand to help people understand meanings better.

By the way... You should be glad "atheist" and "agnostic" don't require capitalization, because if they did, then that would mean they are religions or ideologies with leaders and tenets. And you don't want to keep having to explain that they aren't, do you?

It is a useful argument against the idiots that claim that atheism is a religion

I do not capitalize christian or christmas. Spell checks everywhere go bonkers!

Because throughout history, Christians had a habit of killing non believers. The capitol C, is a sign of respect, though we don't really mean it. IE "I never said Christians were stupid. I respect Christians! Look, I even used a capital C."

That red squiggly line does have a very powerful affectation, yes? LOL!!!

Consider yourself lucky you remember this rule of grammar in your writings. I am sure I break the rules constantly in many ways small and large and I hate it.

I've stopped using caps for religious names, for it makes no sense.

I don't capitalize any religious word.

No capitalization is needed

I don’t capitalize it, at least not on purpose. The darn phone has a hussy fit when I don’t, though.

On a serious note.... Who cares?

One, being a religion, is a proper noun and the other is not. Keep sticking it to the man or whatever by not capitalizing it I guess.

i don't capitalize anything unless i am EMPHASIZING or unless i am on my phone and powerless before its infinitely unwise autocorrect.

g

I refuse to capitalize christian or god

I guess because cristians think he's the big guy in the sky, I always use a small g, god deserves no respect.

Xians love to capitalize everything: God, Jesus, Bible, Scripture, Crucifix, His, Him, Word. I think they believe it makes gawd happy so He doesn't smite them.

See what I did there?

I deliberately make sure to use lowercase lettering for LOTS of words that
I've always been told should be capitalized.
I've actually had people tell me I'm "supposed" to use capitals, and I
laugh at them.

Because Christian is a proper name and atheist isn't? I don't capitalize god.