It is axiomatic among scholars that Middle Arabic texts, including especially Judaeo-Arabic and Christian Arabic corpora, attest non-Classical manifestations of numerous features, including especially nominal case inflection. However, very little work has been done exploring the nature of the distribution of the various manifestations of nominal case in Middle Arabic corpora. This paper studies several manifestations of nominal case, including the ‘five nouns’, as well as nouns that end in *-āʾ, in Arabic Gospel manuscripts from the 9^th – 15^th centuries CE. Contrary to previous grammatical treatments of the feature, the paper shows that in almost every category the case system as reconstructible to Proto-Arabic, and attested in the ʿArabiyyah, is attested in this Middle Arabic corpus. Those aspects of its distribution that differ from the Classical model are patterned and explained here by appealing to well-known processes of linguistic change, often paralleled by other Semitic and Arabic corpora.