Venerable Bede wrote:The Archregimancy wrote:Though, for what it's worth, the Byzantines initially considered Islam to be a slightly eccentric Arian heresy rather than a new religion precisely because of the position of Jesus within Islam; and your average 7th to 8th-century Byzantine theologian was hardly ignorant of comparative theology.
From a purely historical perspective, the separate status of Christianity and Islam may seem inevitable now, but it didn't necessarily seem so inevitable to early contemporaries.
They probably didn't know Islam rejects both the Crucifixion and Resurrection. If they did, they certainly wouldn't see it as simply a heresy.
It's difficult to tell exactly what Byzantines thought of Islam in the 7th century, not least because 7th-century Islam itself is shrouded in mystery. It's only in the last few decades that we've been able to conclude that the Qur'an actually does date to the 630s and 640s (if not even earlier), rather than to the end of the 7th century.
7th-century Byzantine authors did seem to think Islam was closer to Judaism than it was to Christianity. The
Teaching of Jacob, an anti-Semitic screed from the 630s, shoehorns in a rejection of the idea that Muhammad was the Jewish Messiah. The
Apocalypse of Pseudo-Shenute from the 640s 'prophecied' (after the fact) that the 'Ishmaelites' would 'hound the Christians' and seek to rebuild the Jewish Temple. And the Armenian bishop Sebeos wrote in the 660s that Muhammad convinced the Arabs to worship the God of Abraham (and, for that matter, banned eating carrion, drinking wine, telling lies, and committing adultery), and forged an alliance against Herakleios with the Jews, based on their common descent from Abraham.
Certainly in the 7th century, there didn't seem to be much of a suggestion that Islam was a Christian heresy - only that it was an Abrahamic, monotheistic faith with some relationship to Jewish Messianic prophecy. Quite what led the Byzantines to make that latter connection is another question altogether.