[jcranmer]

The best answer may well be some time in the 1500s. Recall that as the era shifts from the late Classical to early Medieval, all of these people are still speaking the Roman language (Vulgar Latin, which evolves into the various Romance languages), following the Roman religion (Christianity), obeying Roman legal codes, and in many aspects, still following Roman customs and experience the same Roman economic and administrative system. In the case of the Byzantines, there is continuous institutional survival until the Fourth Crusade in 1204. Now, what that Roman society looks like in the 500s is very much not the same Roman society we conceptualize of in the 100s, but there is largely no clean break [1]. There is political disunity, but that doesn't necessarily mean that there isn't a recognized commonality of culture (cf. China, where similar major periods of disunity are still categorized as all being part of China).

The major rupture is the Protestant Reformation, where the split between Protestant and Catholic Christianity proves irreconcilable, and results in the end of the notional idea of a unified Christendom. This is also when you start to see an end towards the practice of writing in the literate language of Christendom (i.e., Latin) and instead move towards working in the vernacular, especially in endeavors like scientific research.

[1] The major exception is Britain, where the end of Roman rule is very abruptly realized, and there is a distinct clear horizon between sub-Roman Britannia and Anglo-Saxon Britain. But the British experience is largely the exception, not the rule.

[userulluipeste]

"The major rupture is the Protestant Reformation, where the split between Protestant and Catholic Christianity proves irreconcilable, and results in the end of the notional idea of a unified Christendom."

The Christendom ceased to be unified a whole lot earlier: https://en.wikipedia.org/wiki/East%E2%80%93West_Schism

[wahern]

The Church of the East split in the early 5th century, followed later that century by the Oriental Orthodox Churches. Altogether they may have been larger than the Roman (Latin and Greek/Byzantine) Church.

Armenia became a Christian state before the Roman Empire did, and Ethiopia not long after Rome did. (Both churches now part of Oriental Orthodoxy. Another state church around the area of Sudan emerged, too, the last of whom disappeared only in the 1600-1700s.)

The growth of Christianity in the Persian Empire created a major source of friction between the Roman and Persian Empires (the latter was nominally Zoroastrian, at that time also a proselytizing religion), creating space for the emergence of Islam, which would later lead to the end of both empires and the conversion of millions of Christians and Zoroastrians.

Christianity was never coterminous with the Roman Empire. It just seemed that way from the perspective of European history and culture. European Christianity eventually forgot about those other Christians (I'm not sure if the reverse was true, though). Relative to modern Protestantism, all of these churches have near identical theology, Roman Catholicism included. Which perhaps bolsters the point about Protestantism representing a significant break in the European historical narrative.

[jcranmer]

Yes, there's a rupture between Eastern and Western Christianity, but Western Christianity still accepts the authority of the pope to speak on behalf of Christianity, and there's still a sense that they're still part of the same Christendom, just disputing who is going to come out on top. You might compare it to the modern "One China Policy" in that both Taiwan and China see themselves as the legitimate government representing all of China. Note that the Holy Roman Emperor and the Byzantine Emperor both titled themselves as Emperor of the Romans--they're still claiming heir to the same unified Christendom.

(And also note that the latest Byzantine Emperors repeatedly tried to mend the schism to secure Western aid in stabilizing their empire.)