Thread by @shahanSean

In Q Ḥašr 59:23, God is called: al-muʾmin al-muhaymin. The first name, muʾmin|مؤمن, usually means “believer” but in this context probably means, “faithful,” or “granter of safety (ʾamn).” But what about the second term, al-Muhaymin?

If you've studied Aramaic, the answer seems pretty straightforwoard: al-muhaymin is easily recognizeable as mhaymen|ܡܗܰܝܡܶܢ|מהימן.
It means the exact same thing as al-muʾmin, i.e., “believer” or “faithful”.

Theodore Nöldeke (1836-1930) noted the Aramaic background to al-Muhaymin, but surprisingly he did not find the word to be suitable adjective for the divinity, saying "there is not meaning in Aramaic which may be applied to God."

By contrast, the grammarian Abū Isḥāq al-Zajjāj (241-311/855-923) cites an anonymous opinion that al-muhaymin “is one of the names of God in the ancient scriptures (fī l-kutub al-qadīmah).” Who's right?

Deut 7:9 refers to God as “The faithful/steadfast God (haʾel hanneʾeman) who observes his covenant” (cf. Isa 49:7, Jer 42:5). The Hebrew hanneʾeman was almost always translated into mhaymen|ܡܗܰܝܡܶܢ|מהימן.
It had become a standard name for God in Palestinian Aramaic and Syriac.

So, as it turns out, al-Zajjāj's anonymous source was correct: it's a name for God in the older scriptures The words muʾmin and muhaymin are basically synonymous; the latter merely sounds a little more exotic in Arabic, like when an English speaker calls the devil "El Diablo".

Does any of this help out with the meaning of a muhaymin ʿalayhi in Q 5:48? Perhaps. One may find a parallel usage in the Syriac Gospels. Matt 25:21, 23 conveys a parable about servants ...

it speaks of two servants; each is “a good and faithful servant (ʿabdā tābā w-mahaymnā| ܥܒܕܐ ܛܒܐ ܘܡܗܝܡܢܐ)” because he is “faithful over a little (ʿal qalīl mhaymen| ܥܠ ܩܠܝܠ ܡܗܝܡܢ ).” Is the Qur'an similarly "muhaymin ʿalā" the past scripture, as in faithful to it?