There is also the superstition of the Ishmaelites which to this day prevails and keeps people in error, being a forerunner of the Antichrist. They are descended from Ishmael, [who] was born to Abraham of Agar, and for this reason they are called both Agarenes and Ishmaelites… From that time to the present a false prophet named Mohammed has appeared in their midst. This man, after having chanced upon the Old and New Testaments and likewise, it seems, having conversed with an Arian monk, devised his own heresy. Then, having insinuated himself into the good graces of the people by a show of seeming piety, he gave out that a certain book had been sent down to him from heaven. He had set down some ridiculous compositions in this book of his and he gave it to them as an object of veneration.
– St. John of Damascus, Concerning Heresies
Islam promises a worldly-dominated blissful happiness, the fulfillment of all desires and sensations of the faithful, but also a mental blissful happiness after death for the followers of Allah. Western Christians better understand this blissful happiness, despite the ascetic life of the Orthodox Church which ascetically aims at overcoming blissful happiness. The same commonalities are observed in other issues, such as the issue and source of Faith. For the Muslim the Koran is the revelation of God, and the redemption of the believer depends on the book and its reading. This mindset is also found among Western Christians, for whom the Bible is the word of God and the only source of faith, which is why [Protestant] Westerners better understand the Muslim perception of Revelation rather than the Orthodox, for whom the Gospel is not a Revelation but words about the Revelation
– Metropolitan Hierothos of Nafpaktos
Show me just what Muhammad brought that was new and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached. God is not pleased by blood – and not acting reasonably is contrary to God’s nature. Faith is born of the soul, not the body. Whoever would lead someone to faith needs the ability to speak well and to reason properly, without violence and threats. . . . To convince a reasonable soul, one does not need a strong arm, or weapons of any kind, or any other means of threatening a person with death.
– Byzantine Emperor Manuel II Palaiologos, 26 Dialogues with a Persian