As mentioned previously, I'm a fiction writer. As a writer, I've developed storylines in a significant number of different universes, often with multiple stories in each universe. Most of these universes are original - they are not based on the work of anyone else; however, some are fan fiction of existing universes, either real or imagined. Some of the existing universes my stories reside in are the Starcraft universe (most forming a collective work known as Apocalypse), the Warcraft universe, and the real world (such as The Mission, Eve of Tomorrow, etc.).
The Warcraft storyline (as of yet unnamed) consists of quite a few stories ('books', if you want to think of them that way) about the events surrounding a particular religious organization, with three characters playing the lead roles - Ambrose, the founder, and his adopted children (orphans from the events of Warcraft III), Julius and Nadia. The organization was formed for a specific purpose in the storyline: to annihilate the Scourge on Azeroth, and to banish the Burning Legion from Outland. This storyline was developed after Warcraft III was released (obviously), but before The Frozen Throne was released (I was rather annoyed to see the Scarlet Crusade in World of Warcraft, as among more obvious reasons red was the color I was using).
I suspect I'm giving much more information than is necessary for the purpose of this post. Anyway, this organization worships a god, and obviously such an entity needs a name. While some parts of the storyline (as with most other storylines) developed more gradually, I knew the name I would use from the very beginning: the name was to be derived from 'agnostos', the Greek word meaning 'unknown' (same root as English 'agnostic').
This name was chosen for several reasons. At the time Ambrose and god first met, the god had been nearly inactive since ancient times; no followers, and virtually no written records, remained. So clearly god was very much unknown to the world at that point. The second reason is similar: Ambrose giving god this name refers to some of the particular, humorous phrasing in the first conversation between Ambrose and god (when Ambrose still didn't know who god was), and reflects the somewhat tongue-in-cheek relationship between the two. I believe there was another reason for choosing 'unknown' as the name, though I can't think of it off the top of my head. The final reason for choosing this word, as well as the reason for using Greek, are left as an exercise for the reader (good luck with that). However, unforeseeable developments made the choice of Greek work even better than I initially expected.
As I'd decided from the beginning to use Greek (specifically Ancient Greek), it would seem logical to also put holy texts or mantras in Greek, as well. There's just one problem... while I know enough Latin to at least be able to make coherent text with the aid of a dictionary, I know little more about Greek than the fact that it's grammar kind of resembles Latin's (I know a bit more now, but still not enough that I'd try writing text in it). This led to the decision to use Latin as the "present day" (the time of the storyline) holy language.
It wasn't difficult to rationalize this choice, taken from real-world history. There's clearly some analogy between this fictional organization and the Catholic Church, the latter often using Latin for holy texts and common sacred phrases. Similarly, much of the New Testament was written in Greek (in fact, I believe the name Jesus itself is Greek - the original Hebrew version is Joshua). Thus there was real, historical precedent for Greek being the holy language "in the past", and Latin "in the present" (never mind the fact that technically Latin was also spoken at the time of the New Testament, as it was the language of the Romans, who ruled during that time, and that Latin was or nearly was a dead language by the time the Catholic Church was formed).
This idea was then refined after I read a portion of Negima! For those not familiar with the series, it involves, among other things, battles between wizards of different schools of magic, from the Harry Potter type of western magic to ancient Japanese sorcery. I found the fact that different languages were used for spells of different schools (e.g. Latin, Japanese, Sanskrit) particularly interesting. As it turns out, the author used Latin for most western incantations (not surprising); however, for added force, the author decided that for older, more devastating western magic (somehow a common theme in fantasy seems to be that the older magic is the stronger it is), he used Ancient Greek. I had previously planned on using Latin for the names of the special abilities of Ambrose, Julius, and Nadia; this gave me the idea to use Ancient Greek for the ultimates of the three.
*Ahem* getting back on track... I ultimately decided on 'Agnos' as the name of god. Agnos is, among other things, associated with fire. Among other examples, the Virifeges Dei ("Fangs of God"; yes, I did coin that word, myself, for this purpose), the tangible manifestation of Agnos' power, appears as a white flame. One word in Latin for fire is 'ignis' (same root as English 'ignite'). Thus 'Agnos' is the result of a dual derivation: deriving the name from Greek 'agnostos' and from Latin 'ignis', words in both holy languages which were relevant to Agnos' character. I think that turned out impressively well.
This brought up a rather sizable question: how the heck am I supposed to decline (see meaning 4a) that? 'agnostos' is a Greek word, and 'agnos' is a Greek analog; if the two were declined the same, you'd get Agnos in the nominative, Agnou in the genitive, Agno (or something like that; Angoi? Agnoh?) in the dative, Agnon in the accusative. Only, I'm writing Latin, not Greek. -os is NOT a typical Latin declension suffix.
Well, there are quite a few possibilities, based on theory alone. I could decline it like 'dominus' ('lord') but with an altered nominative: Agnos, Agni, Agno, Agnum, Agno (in the ablative), and Agne (in the vocative). I could also decline it like 'honos' (older form of 'honor', identical in English and Latin): Agnos, Agnoris, Agnori, Agnorem, Agnore, Agnos. I could even use the fourth declension.
Fortunately, some research revealed that Latin already has a mechanism for borrowing words from Greek analogous to 'agnostos': the first of those options listed: use the first declension, but use the -os form for the nominative. Ironically (and in a typical example of me overthinking a problem), there already exists a word 'agnos' in Latin - a type of tree - and it's declined in this way. While I don't mind that so much, there's one more coincidental occurrence I'm not so sure about (and in fact only just noticed): Agnos and 'agnus' ('lamb', e.g. Agnus Dei - Lamb of God) are identical in most cases. That wasn't intentional, for anybody wondering :P