I think Bitcoinist can be good for someone that went through a professional study of bitcoin. But there is nothing called Bitcoinist as of now. I will prefer Bitcoiners for those that are holding bitcoin and for those that are bitcoin enthusiasts.

Religious? lol!

-er is for the action of doing that, it's used only for verbs  "the one that does"  drive driver , paint painter , mineminer
-ist is for one that works with something, both nouns and adverbs, so you don't have bycler/motorer  but bicyclist and automobilist motorist

You have the verb to teach you have teacher, you teach physics (noun) then physicist

No!

No!

Don't forget we have "pharmacy" and we have pharmacist which is not a religious something but also an occupation.

We also have archaeologist, neurologist, name it...

In my opinion, bitcoiner, and that is because most of the community use bitcoiner and not bitcoinist, i don't think i have seen someone use the latter. However, bitcoinist may also be correct as the suffix -ist is added to words in order to form a noun, so it could be added to the word bitcoin, to form the noun bitcoinist, which would mean somone who uses bitcoin.
I don't think there is a specific word that denotes this, but bitcoiner also sounds correct here, no?

Bitcoiner is more suitable, you can find it in many unofficial dictionaries. I always mention someone who interested with Bitcoin are Bitcoiner or Bitcoin enthusiast, I never mention "Bitcoinist", it just sound not right and strange.

1. https://en.wiktionary.org/wiki/bitcoiner
2. https://www.urbandictionary.com/define.php?term=Bitcoiner
3. https://crypto.ro/en/dictionary/bitcoiner/

Discussing about this, it makes me remember with this user CryptopreneurBrainboss, so another alternative could be Bitcoinpreneur.

Hmm pretty Well, among the two you have mentioned, Bitcoiner sounds like the better one because it kind of matches the action, someone who either uses or makes investments in Bitcoin can be called a Bitcoiner. Even though I feel like there should be a different word for this because it doesn't sound like the best.

Bitcoinist sounds completely off, it almost makes it sound like the person is running Bitcoin or has control over it, or teaches Bitcoin, etc., but it doesn't sound like it can be used to call a person who either uses or makes investments in Bitcoin.
So, conclusively, I would rather go with Bitcoiner than Bitcoinist because the first word sounds much better for this instance.

We probably need to consult a lexicographer or an etymology specialist or any language oriented professional regarding this one because this isn't something that's really easy to talk about or decide despite being such a simple question, that's just my opinion though so take it with a grain of salt. Another thing I'd like to point out is that regarding this issue of which one is the right one, I'd probably say that you're better off speaking what you're most comfortable with, it's not like the Gods of language is going to be striking you down when you say what you think is the wrong one, language is a construct so there's really no rules about pronunciation, just say what you're most comfortable and you'd eventually notice that people don't give a fuck about how you pronounce it.

I would go with Bitcoiner, I got used being called that way or crypto earner, I don't really care much about it.
For me it is all the same, we are earning in crypto or working with crypto currency, that's all that matter to me, I don't care what other people thinks or call me, I am earning no matter what they say.

It is only the professionals in English language that will be able to tell the correct one to use but basically, I do not care about English language because I did not study it and so I believe that English language is for effective communication, and communication is complete when the person you are speaking to understands you. If I use English language wrongly and the person, I am speaking to is still able to understand me, I have still communicated. Bitcoiner or bitcoinist does not really matter which is the proper language for use, if it is too confusing, you can simply say Bitcoin enthusiast.

Having gotten used to the word bitcoiner overtime, the word bitcoinist sounds strange to my hearing even though the
ist makes some sense as found in other conventional words.

Perhaps as we progress and bitcoin gains more larger acceptance even into the academic community to be studied as a discipline or field with different scopes (in theory and technical), we will be addressing academicians that study bitcoin to specialized in bitcoin as bitcoinist.

In as much as for now bitcoin is not studied as a an accredited course in schools (am open to correction if its already has),  I think it's fair  stick to the word bitcoiners as users instead of bitcoinist as one who specializes in the study as a profession.