The following line occurs in an early Christian commentary on Genesis:
Quid est enim aliud hodieque gens ipsa, nisi quaedam scrinaria Christianorum, bajulans legem et prophetas ad testimonium assertionis Ecclesiae, ut nos honoremus per sacramentum, quod nuntiat illa per litteram?
The word scrinaria puzzles me. I have not found it in any Latin dictionary, but I suspect it is related to either scrinium (=a desk or chest) or scriniarius (=keeper of the scrinium).
I agree with Sebastian Koppehel that scrinaria in the quoted passage is a feminine singular noun derived from scrinium and the suffix -arius, -aria, -arium; this means scrinaria = scriniaria.
The omission of i after the n has no explanation in terms of the standard Latin rules of word-formation, and does not seem to be significant with respect to the meaning of the word. The spelling scrinaria might be due to accidental omission of the letter i when writing the word, or an irregular alteration in the form of the word.
This suffix is a bit funny. It has the form (or morphology) of an adjective ending, but many words ending in -arius, -aria, -arium are used more frequently as substantive nouns than as attributive modifiers of other words. The masculine forms in -arius (genitive singular -arii) usually are used as the names of occupations or professions, and are highly productive in this function: thus, ferrarius = "blacksmith", gallinarius = "keeper of poultry", etc.
The gender of a form ending in -arius, -aria, -arium can sometimes be explained by interpreting it as an adjective in agreement with an "implicit" head noun: thus, aurarius used as a noun "goldsmith" could be interpreted as standing for something like "(artifex) aurarius" or "(faber) aurarius". But it isn't always clear what the implicit accompanying noun would be.
This can make the interpretation of forms a little tricky.
The suggestion in Sebastian Koppehel's answer that scrinaria is feminine in agreement with gens seems plausible to me, and is in agreement with the translation "a keeper of the records" cited in Tyler Durden's answer. This is also the explanation given by Paula Fredriksen in "Anti-Judaism and Early Christianity" (Marginalia, December 9, 2013, review of David Nirenberg’s Anti-Judaism: The Western Tradition), where the word scriniaria is glossed as "[female] librarian". Fredriksen also points out there that "desks" is a mistranslation of the Latin.
It seems Fredriksen has also rendered scriniaria as "bookslave" (in Augustine and the Jews: A Christian Defense of Jews and Judaism, 2010), according to Figuring Racism in Medieval Christianity, by Lindsay Kaplan, 2019 (page 33). That translation seems to be intended to convey the overall meaning of the specific passage where the word occurs (Kaplan, page 187, provides a quote where Fredriksen (2010, page 320) argues that Augustine here is portraying the Jews as "servile book caretakers"). I don't think "bookslave" is as accurate a translation of scrin(i)aria when considered in isolation: words ending in -arius/-aria could refer to slaves, but they did not systematically denote slaves as opposed to other types of craftsmen or workers.
It seems a bit tricky to give a precise definition of the word scrinium. The basic literal sense seems to be chest; this can have the sense of letter-box or box of records. According to The Diplomas of King Aethlred 'the Unready' 978-1016, by Simon Keynes (1980):
The word scriniarius is ambiguous, denoting either 'archivist' or 'custodian of books' from the primary meaning of scrinium ('chest, for storage of books') or 'keeper or custodian of relics' from its secondary meaning ('reliquary' or 'shrine'). It was apparently the second meaning of scriniarius that was intended by the Abingdon glossarist, with an extension to cover the guardianship of precious things in general.
(page 147)
In the context of the passage quoted in your question, the "archivist" or "keeper of books/documents" sense surely fits, but perhaps it would also fit to view the word as having some connotation of "keeper of treasure/precious things".
I think an alternative possibility is that scrin(i)aria could refer to a place or location (used metaphorically in this case). Some nouns ending in -aria have that kind of meaning, such as auraria "gold-mine" or libraria "collection of books". In that case, scrin(i)aria would have a sense more like "collection/archive of scrinia". But this seems less plausible to me than the interpretation given by Paula Fredriksen and Sebastian Koppehel.
Some nouns ending in -aria have multiple possible meanings; e.g. see the Dictionary of Medieval Latin from British Sources's definition of cameraria (available via Logeion).
It seems obvious to me that it is supposed to be the feminine form of scriniarius (which can also be parsed as a first/second declension adjective, “pertaining to the scrinium” or something to that effect). Thus it also means “keeper of the scrinium.”
And why use a feminine form? Because gens is feminine. I'm not sure there is a rule that you could not possibly say ea gens est quidam scriniarius Christianorum etc., but there is certainly ample precedent for using the feminine form where applicable, e.g. Cicero: Historia vero [⋯] magistra vitae, nuntia vetustatis (de Oratore 2, 36), [lex perpetua et aeterna], quae quasi dux vitae et magistra officiorum (de Natura Deorum 1, 40; note: no feminine form of dux exists, so it gets used as is), and this very impressive period: Vitae philosophia dux, o virtutis indagatrix expultrixque vitiorum! [⋯] tu inventrix legum, tu magistra morum et disciplinae fuisti. (Tusculanae Disputationes 5, 5)
It is a metaphorical term meaning recordkeepers in this context.
There are three possibilities for the derivation of this word:
(1) It is an invented word meaning a chest of records. So, in other words, in the same way that an aulularia is a pot of gold, a scrinaria is a chest of records.
(2) It is the plural of scrinarium which is an error for scrinium (a letter case), meaning metaphorically the Jews are the "letter cases" for the Christians
(3) It is an (otherwise unattested) feminine singular of scriniarius (the keeper of letter cases).
In the last two possibilities, the text contains an error of some kind, because it should have been scrinia, not scrinaria, or alternatively it should have been scriniaria. In other words, either the author is writing scrinarium / scrinaria instead of the correct scrinium / scrinia, or the author is mistakenly writing scrinaria for scriniaria. Scriniarius cannot be intended because the plural of scriniarius is scriniarii.
A scrinium is a case to hold letters. For example, here is a quote from Sallust:
Flaccum praetorem scrinium cum litteris, quas a legatis acceperat, eodem adferre iubet.
(He ordered the praetor Flaccus to bring to the same place the letter case together with the letters which he had taken from the envoys.)
As I originally read it, the author is characterizing the Jews as the "letter cases" of the Christians, meaning that they keep the records of the Christians and record their history. It is a metaphor. Of course, one can also imagine that the intent was to write scriniarii, which would be the plural of scriniarius, the keeper of the letter cases. However, this is unlikely because of the neuter plural ending, which clearly indicates the intention to refer to an object, not a person. The first possibility listed, the coined word scrinaria (a letter chest) is a very viable possibility because in that case there is no typo.
In any event, the ultimate meaning is clear: the author meant recordkeepers.
I will quote Louis Gaussen (1790-1863) comments on this passage in his book "Canon of the Holy Scriptures":
But no matter still the oracles of God are confided to you and we may say of you at this day what Augustin wrote a thousand years ago what is the nation of the Jews even in our day but as it were a keeper of the records for Christians carrying everywhere the law and the prophets as a witness of all the Church affirms (Et quid est aliud hodieque gens ipsa Judaeorum nisi quædam scrinaria Christianorum bajulans Legem et Prophetas ad testimonium assertionis ecclesiae.)
(As long as we are talking this word be aware there is also the term scrinaria palatii which were special officials reporting I think to the Roman superintendant of the treasury, but that is a completely different use of the term that does not apply here.)
