Medieval Christian polemicists calibrated Islam’s threat by classifying it as pagan or heresy.

Evidence

Peter then gives a rhetorical objection to this line of argument: one could say that the Saracens were pagans (ethnici or pagani) rather than heretics. For did not John define the “many Antichrists” (which, for Peter, means heresiarchs) as those who “went out from us, but they were not of us” (1 John 2:19), in other words as those who had been part of the church and had broken away from it? Peter notes that, like heretics, the followers of Muhammad adopt parts of the Christian faith and reject other parts, while they also follow some rites that seem to Peter “pagan.” Like certain heretics, Peter says, Muhammad “wrote in his impious Koran” that Christ was born of the Virgin Mary, lived without sin, and performed miracles; like the Manicheans, the Saracens deny his death. Like the pagans, on the other hand, they reject baptism, the mass, and the other sacraments. Heretics or pagans, “choose whichever you like” (Contra sectam, §14). He asserts that pagans should also be opposed by written polemic; here, too, he lists the names of illustrious church fathers who attacked paganism in their writings. Peter himself generally prefers to consider the “Mahometan error” as a heresy. — John V. Tolan, Saracens, 851