When considering antiquity, late antiquity and the early Middle Ages, it is worth pondering the basis for terms we may regularly use in our narratives and analyses of those periods and whether they are used or reflected in the sources of those times. For example, to my knowledge, the exact formulation 'Western Roman Empire' (Latin: e.g. imperium Romanum occidentale) is not used in an ancient source from the time it existed (up to 476/480 CE). On the other hand, the term is suitable in describing the division of the realms of the Roman Empire in late antiquity into two imperial courts that were de facto independent of each other even as there was nominally still one empire.
Another term of interest to consider is "Jewish Christians." I came across this matter in an academic article I recently had to translate into Arabic. The article, published in 2021, entitled "The Deadlocked Debate about the Role of the Jewish Christians at the Birth of Islam" and written by the Spanish scholar Francisco del Río Sánchez, discusses some of the academic debate about whether "Jewish Christians" had an influence on the birth of Islam. He notes that there is no precise and agreed definition of this term but that it has been used to refer to those individuals or groups who believed Jesus was a prophetic, messianic or even divine figure but observed the Torah in part or in full.
While he notes that there is no evidence for the existence of "differentiated and hidden Jewish-Christian communities" in Arabia or the Middle East during the time of the Prophet Muhammad, he notes nonetheless that some sort of terminology along those lines may be useful to describe those who were located somewhere in between the poles of 'conventional' Judaism and Christianity, and existed in the Middle East even during the later years of the Umayyads (early to mid eighth century CE). It would then be worth considering how "mixed sensibilities, practices and beliefs" in this very large middle ground might be reflected in Islam's foundational texts like the Qur'an.
With regards to the specific term "Jewish Christian," Sánchez notes that there is a such a formulation in a source from the late seventh century CE: namely, in the work 'De Locis Sanctis' ("Concerning the Holy Sites") by the Scottish Gaelic writer Adomnán (Adamnanus). In the work, Adomnán relates a story about a shroud with which Jesus' head had been covered, and a dispute about ownership of the shroud between the "Iudaei Christiani" (ablative plural form: "Iudaeis Christianis," which is the inflectional form used in the text) and the "infidel Jews" (referred to by formulations in the text such as "infideles Iudaei"- ablative plural form infidelibus Iudaeis, which is the inflectional form in the text). The two disputing parties resort to the arbitration of "Mauvias king of the Saracens" (i.e. the caliph Mu'awiya (I?)) who throws the shroud into the fire in a bid to see which party the shroud rightfully belongs to. The shroud then miraculously comes out of the fire unscathed and descends on the 'faithful,' who rejoice and place it in the storage of the church.
"Iudaei Christiani" can literally be translated as "Jewish Christians." Yet on reading the whole story myself, it seems to me that the meaning of the formulation here is unlikely to be the one alluded to in the second paragraph of this post, but perhaps just means Christians of Jewish ethnic origin (i.e. Jews who formally converted and/or their descendants), or Christians of Jewish and non-Jewish ethnic origin who are now deemed the 'real Jews' in having believed in Christ and thus undertaken the new covenant as opposed to the Jews who have rejected Christ. Yet these readings are by no means certain, and thus as Sánchez concludes it seems that "Jewish Christians" in the sense alluded to earlier is a modern construction.
Since the whole story as related in Adómnan's work (De Locis Sanctis 1.9) is interesting, I have decided to translate it in full here, while providing the original text, the edition of which is taken from Itinera Hierosolymitana Saeculi IIII-VIII (ed. Paul Geyer, 1898):
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