We are used to thinking of Islam as a religion separate from, though related to, Judaism and Christianity. Indeed, even many Christians appeal to a common heritage between these three faiths, all being so-called "Sons of Abraham." But it would appear that Islam is not simply a separate faith that grew up out of its own soil. Rather, it would appear that it is a form of Christian heresy; and, indeed, this was how it was viewed by the Christians who knew Islam in its earliest days.
Let's first look at Church historian Sozomen. Writing in the early part of the fifth century, about two hundred years prior to the rise of Islam, Sozomen notes:
This is the tribe which took its origin and had its name from Ishmael, the son of Abraham; and the ancients called them Ishmaelites after their progenitor. As their mother Hagar was a slave, they afterwards, to conceal the opprobrium of their origin, assumed the name of Saracens, as if they were descen