When having two continuous adjectives, how should I choose what will be first in a sentence? or it doesn't matter?
For example:
He's "religious Christian"
or
"He's Christian religious"?
I want to say that he's deeply religious and goes to church twice a week. But both are adjectives: religious and Christian and I'm not sure about the order in such case.
The words Christian and religious both can be either adjectives or nouns. Both of your sentences make sense if you add in the indefinite article, but they mean different things (and the second one uses somewhat specialized terminology).
Using Christian as a noun and religious as an adjective:
He's a religious Christian.
This means that he is a Christian (a noun - scroll down at the dictionary link you gave in your question and you'll see that there's a noun definition) who is more than just casually Christian. More often, the adjective "devout" is used to mean "very observant."
Using religious as a noun and Christian as the adjective:
He's a Christian religious.
This is the one that's a bit more unusual. As a noun, a religious is someone who is a member of a religious order, like a monk. This is honestly a fairly rare usage of this word that even a lot of native English speakers would be unfamiliar with, so if you want to say that someone is a monk and you're not speaking to people who deal with members of religious orders on a regular basis, you should probably just say, "He's a Christian monk."
While I don't dispute Canadian Yankee's excellent answer, if you specifically wanted to know which order these should go in if you want to use them both as adjectives, then the answer is, it depends on what you want to emphasize.
The more common order would be this:
He's a religious Christian man.
That means he's a Christian man who is very devout.
However, if you want to emphasize that he's a very devout man whose flavor of religion is Christian, you could use the opposite order:
He's a Christian religious man.
That is less idiomatic, but might be appropriate in some contexts.
Honestly, neither of these sounds like proper English so I'm not sure if it's best to answer the question itself (the order of adjectives) or help rephrase the sentence.
We would tend to say, in English, for what you describe, simply "He's a devout Christian" with one adejctive and one noun.
To address the original question, English has a fairly strict set order for adjectives depending on the type of adjectives used. This is something which is taught to foreign learners of the language but native speakers tend to acquire naturally.
Quantity, quality (opinion), size, age, shape, colour, proper adjectives, e.g. French, purpose, qualifier
Changing the order can affect the meaning of the sentence.
While the other answers are good for your specific sentence, in general, in English we don't say
[noun] is [adjective] [adjective] .
we say
[noun] is [adjective] and [adjective].
So it would be grammatical to say "He is religious and Christian." or "He is Christian and religious." but neither "He is religious Christian" or "He is Christian religious" will sound right to a native speaker.
Other examples:
"The tablecloth is white and lacy." is correct and "The tablecloth is white lacy." is not.
Other posters have talked about the order of cumulative adjectives, but I don't think that really applies when the adjectives are the complement of the copula. A copula can't take cumulative adjectives:
"It's a red coat" ✅
"The coat is red" ✅
"It's an amazing red coat" ✅
"The coat is amazing red" ❌
"The red coat is amazing" ✅
So while you can say "The religious Christian man", you can't say "The man is religious Christian".[1] There are several other options:
Change the adjectives to coordinate adjectives: "He's Christian and religious"
Use "Christian" as a noun: "He's a religious Christian"
Turn "religious" into an adverb: "He's religiously Christian"
You can also use another word, such "He's a devout Christian".
[1] There are cases such as "He is fake Christian", that might appear to be exceptions, but a full analysis of those usages is beyond the scope of this question.
The order matters as explained here: https://dictionary.cambridge.org/grammar/british-grammar/about-adjectives-and-adverbs/adjectives-order
Quoting from the above source
Adjectives which describe opinions or attitudes (e.g. amazing) usually come first, before more neutral, factual ones (e.g. red):
She was wearing an amazing red coat.
Not: … red amazing coat
In your example, religious is an opinion and Christian is neutral/factual and hence the order is religious Christian.
Maybe substitute Christian -> American (or any nationality of your choice) and religious -> patriotic.
You'll then see that 'He's a patriotic American' reads much better than 'He's an American patriotic`. The second form might be common in poetry, but I doubt you'll see it much in prose.
If you're thinking it of a sentence along the lines of 'He's an American patriot', then the equivalent here would be 'He's a christian (evangelist? / apologist?) ...' - not sure what the equivalent word for 'religion' is. Certainly isn't 'religionist', even if that's a valid word.
"Christian" can either a noun or an adjective.
For example:
NOUN: He is a Christian.
ADJECTIVE: Fighting is not a Christian thing to do.
"Religious" is normally only used as an adjective. (Other answers and subsequent comments have noted that the word can be used as a noun but only in certain denominations, and notably faiths that historically use Hebrew or Latin as a language. I would suggest that this be disregarded if your goal is to be understood by most English speakers rather than appeal to a particular faith)
Your stated goal is to describe someone as deeply religious.
You describe using an adjective, and it would be incorrect to say:
He is a religious.
Therefore by the same reasoning it is also incorrect to say in English "he is a Christian religious".
The following are grammatically acceptable and will be understood by most English speakers regardless of their religious views:
Debates about what is / is not correct on other answers seem to be about differing religious doctrines, not common English usage.
To be understood by most people, keep it simple. You may be better using an alternative adjective to describe the person's devotion to Christanity, such as:
This would also address the issue that "a religious Christian" is technically a tautology - that is repeating the same thing twice, like saying "a clever genius".
