That time of year again (
https://gbatemp.net/threads/do-you-believe-in-god.405333/ ) huh...
Anyway "Certain religions have been changing their doctrine to fit modern society to make themselves more appealing. Personally, I am not fond of this approach, because you shouldn't have to change the fundamental prospects of your religion to get somebody to join it."
They have been doing it for thousands of years. The Christianisation of Scandinavia being a nice case study, though my personal favourite still has to be rag trees.
If you want to get deep in stuff likely to have conspiracy yelled at you then the plausibility of one Mr Jesus being able to meet John the Baptist is questionable, and leads on to one wondering how much of it was an amalgamation.
The umbrella terms for religions themselves are odd. Someone calling themselves Christian has been around for a while now but the divisions between the Roman Catholics and various protestant movements (see also much of the history of Europe and parts thereof, much of the history of what might be dubbed the Islamic world and parts thereof, Indian history, a fair few notable moments in US history, and the list goes on) makes the umbrella term as a means of self identification tricky (most times you see it before about 1950 then it will tend to mean "my branch").
Going further then why does a religion have to be fixed? I know a lot do (see also something like Quranic Arabic), or have foundational aspects which today fly in the face of what we know about the universe. On the flip side go to the east and religions there blend aspects of whatever new stuff is coming in throughout history (Buddhism's trek across Asia and the Orient being good for this).
I find the fixed thing often leads to the discouragement of challenge and reasoning in certain areas. That I can not abide.
An interesting philosophy I saw the other week was someone attempting to split religion's dogma from religion's societal implications (a sort of religious pragmatism). Normally I would have seen it in the "you only go to church to see what others are doing and drum up business" type things, which are not invalid criticisms, and possibly in the "culturally [blah]" (culturally Jewish and culturally Islamic being where I would see it phrased as such, though the latter can be "apostasy is not well liked in my community so...", and culturally Christian is a somewhat recognised concept even if it is not said as such.
As for me if I was not already obvious then in the words of a great song "Started out with nothing, still got most of it left". I have read all the big books/collections of beliefs of all the major efforts, many of the minor ones too. This mainly as they are rather important from a historical and cultural perspective, occasionally also sociological and legal perspectives.
I don't mind religion as long as others don't constantly preach it.. or constantly attemp to bring it up in coversation such as "As a christian I...." I mean seriously... how many people would be annoyed if every conversation I entered "As an Atheist..."
Linguistically it does go back a little way and used to have more commonly used options; poke around a book search with a phrase like "as a humanist" and you can go back quite far, I got some stuff in the late 1700s and 1800s with that.
That said the modern implications and usefulness are such that I am certainly not going to call your dislike of the concept untenable. I have myself also found such things to be the basis of a fallacious argument (commonly argumentum ad populum)