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For a graphic design for a client I want to render the words "There is no God but the God" into Chinese in as few characters as possible.

It is of course a translation of the Islamic "La Ilaha Illallah" ("There is no God but the God") . And whereas Muslims say "物非主，惟有真主", that's 8 characters long and a huge statement and not easy to fit in my clients design, so I came up with this instead:

無神除神

Notes:

I chose 神 for "God" despite other names being more commonly used in Abrahamic Religions.

The Arabic word Ilah (root of Ilaha and Illallah in the phrase "La Ilaha Illallah") is a generic word for God in Arabic, hence my choice of 神 over other words like 主 or 帝 which are more specific and would be better translations of the Arabic word "Rabb" ("Lord") I'm my opinion.

I know from this question that 神 is also (if not commonly) used for the Abrahamic God. To quote the answer:

"Christian prayers (esp the older translations) refer to god as 主 or 神 （I've seen hand-copied Christian prayers that use both）；but 神 also means "spirit" (if you think of "the holy spirit", it makes sense that 神 is used to refer to god). However, unlike 上帝, the character 神 alone doesn't unambiguously mean "god"."

While 神 "doesn't unambiguously mean "god"" I think, in the context of the phrase "無神除神" it is clear enough.

My reason for choosing 神 came from this site, where it is said (under Terminology):

In Chinese language there is a terminological distinction between 神 shén, 帝 dì and 仙 xiān. Although the usage of the former two is sometimes blurred, it corresponds to the distinction in Western cultures between "god" and "deity", Latin genius (meaning a generative principle, "spirit") and deus or divus; dì, sometimes translated as "thearch", implies a manifested or incarnate "godly" power.

So is 無神除神 grammatical correct and good as a translation of the Arabic "La Ilaha Illallah" ("There is no God but the God") for the purpose of my small graphic design?

So is 無神除神 grammatical correct and good as a translation of the Arabic "La Ilaha Illallah" ("There is no God but the God")

No to both

The shortened form of "除了(真)神之外沒有神" should be "除神沒神". But 除 also mean remove, so "除神沒神" could mean "remove God and there will be no God", Therefore, it is not a clear translation for "There is no god but the God"

"神外無神" (Beside the God, there's no gods) is a better translation

This looks really weird, I can't even guess what 無神除神 means before reading your explanation.

I think the main problem is that, 除, when used alone, usually means get rid of rather than except. So the whole phrase become 'no god and get rid of god'.

As far as I know, there's already a 4-character phrase with similar meaning 認主獨一. I'm not sure if that fits your need.