And the adversary has been striving vigorously to destroy the flock ever since. Islam and Arianism were intended to displace genine Christian faith by offering or forcing acceptance of a counterfeit faith--one that would acknowldge the form of Jesus Christ while steadfastly denying the power. Because that power was made fully manifest in the resurrection, the resurrection had to be denied and denounced as vigorously and thoroughly as possible. Islamic rejection of the crucifixion and resurrection goes beyond ordinary rejection--it is a pathological rejection. And that pathology betrays the true nature and identity of the spirit that inspires it.
True, but the adversary had another, perhaps more subtle approach which was started even while the battle with the Arians was raging in the old Western Empire. This was the Monophysite heresey, which denied that Jesus was human, thus denying any meaning in his sacrifice on the cross, and also denying that Adam's sin had been redeemed, since a non-human God could not pay mankinds debt.
This heresy dominated much of the Eastern Empire at the time of Justinian in the 6th century (the story was that the Empress Theodora was Monophysite). It was also suppressed, but resentment against this suppression weakened the provinces in which it had the most influence (Syria-Palestine and Egypt), so that the resistance to Islam was less than it might have been. The Coptic churches in Egypt and Ethiopia are Monophysite to this very day, I understand.