This essential text for Orthodox Christians regarding Islam is featured as Appendix I in the book, Facing Islam.— the unflinching words of a saint of the Church. See also the Life of St John here.
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Webmaster note: The following passage is from Saint John’s monumental work, the Fount of Knowledge, part two entitled Heresies in Epitome: How They Began and Whence They Drew Their Origin. It is usually just cited as Heresies. The translator’s introduction points out that Fount of Knowledge is one of the most “important single works produced in the Greek patristic period,…offering as it does an extensive and lucid synthesis of the Greek theological science of the whole period. It is the first great Summa of theology to appear in either the East or the West.” Saint John (+ 749) is considered one of the great Fathers of the Church, and his writings hold a place of high honor in the Church. His critique of Islam, or “the heresy of the Ishmaelites,” is especially relevant for our times.
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. They are also called Saracens, which is derived from Sarras kenoi, or destitute of Sara, because of what Agar said to the angel: ‘Sara hath sent me away destitute.’  These used to be idolaters and worshiped the morning star and Aphrodite, whom in their own language they called Khabár, which means great.  And so down to the time of Heraclius they were very great idolaters.
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.
He says that there is one God, creator of all things, who has neither been begotten nor has begotten.  He says that the Christ is the Word of God and His Spirit, but a creature and a servant, and that He was begotten, without seed, of Mary the sister of Moses and Aaron.  For, he says, the Word and God and the Spirit entered into Mary and she brought forth Jesus, who was a prophet and servant of God. And he says that the Jews wanted to crucify Him in violation of the law, and that they seized His shadow and crucified this. But the Christ Himself was not crucified, he says, nor did He die, for God out of His love for Him took Him to Himself into heaven. 
And he says this, that when the Christ had ascended into heaven God asked Him: ‘O Jesus, didst thou say: “I am the Son of God and God”?’ And Jesus, he says, answered: ‘Be merciful to me, Lord. Thou knowest that I did not say this and that I did not scorn to be thy servant. But sinful men have written that I made this statement, and they have lied about me and have fallen into error.’ And God answered and said to Him: ‘I know that thou didst not say this word.” 
There are many other extraordinary and quite ridiculous things in this book which he boasts was sent down to him from God.
But when we ask: ‘And who is there to testify that God gave him the book? And which of the prophets foretold that such a prophet would rise up?’—they are at a loss. And we remark that Moses received the Law on Mount Sinai, with God appearing in the sight of all the people in cloud, and fire, and darkness, and storm. And we say that all the Prophets from Moses on down foretold the coming of Christ and how Christ God (and incarnate Son of God) was to come and to be crucified and die and rise again, and how He was to be the judge of the living and dead. Then, when we say: ‘How is it that this prophet of yours did not come in the same way, with others bearing witness to him? And how is it that God did not in your presence present this man with the book to which you refer, even as He gave the Law to Moses, with the people looking on and the mountain smoking, so that you, too, might have certainty?’—they answer that God does as He pleases. ‘This,’ we say, ‘We know, but we are asking how the book came down to your prophet.’ Then they reply that the book came down to him while he was asleep. Then we jokingly say to them that, as long as he received the book in his sleep and did not actually sense the operation, then the popular adage applies to him (which runs: You’re spinning me dreams.) 
When we ask again: ‘How is it that when he enjoined us in this book of yours not to do anything or receive anything without witnesses, you did not ask him: “First do you show us by witnesses that you are a prophet and that you have come from God, and show us just what Scriptures there are that testify about you”’—they are ashamed and remain silent. [Then we continue:] ‘Although you may not marry a wife without witnesses, or buy, or acquire property; although you neither receive an ass nor possess a beast of burden unwitnessed; and although you do possess both wives and property and asses and so on through witnesses, yet it is only your faith and your scriptures that you hold unsubstantiated by witnesses. For he who handed this down to you has no warranty from any source, nor is there anyone known who testified about him before he came. On the contrary, he received it while he was asleep.’
Moreover, they call us Hetaeriasts, or Associators, because, they say, we introduce an associate with God by declaring Christ to the Son of God and God. We say to them in rejoinder: ‘The Prophets and the Scriptures have delivered this to us, and you, as you persistently maintain, accept the Prophets. So, if we wrongly declare Christ to be the Son of God, it is they who taught this and handed it on to us.’ But some of them say that it is by misinterpretation that we have represented the Prophets as saying such things, while others say that the Hebrews hated us and deceived us by writing in the name of the Prophets so that we might be lost. And again we say to them: ‘As long as you say that Christ is the Word of God and Spirit, why do you accuse us of being Hetaeriasts? For the word, and the spirit, is inseparable from that in which it naturally has existence. Therefore, if the Word of God is in God, then it is obvious that He is God. If, however, He is outside of God, then, according to you, God is without word and without spirit. Consequently, by avoiding the introduction of an associate with God you have mutilated Him. It would be far better for you to say that He has an associate than to mutilate Him, as if you were dealing with a stone or a piece of wood or some other inanimate object. Thus, you speak untruly when you call us Hetaeriasts; we retort by calling you Mutilators of God.’
They furthermore accuse us of being idolaters, because we venerate the cross, which they abominate. And we answer them: ‘How is it, then, that you rub yourselves against a stone in your Ka’ba  and kiss and embrace it?’ Then some of them say that Abraham had relations with Agar upon it, but others say that he tied the camel to it, when he was going to sacrifice Isaac. And we answer them: ‘Since Scripture says that the mountain was wooded and had trees from which Abraham cut wood for the holocaust and laid it upon Isaac,  and then he left the asses behind with the two young men, why talk nonsense? For in that place neither is it thick with trees nor is there passage for asses.’ And they are embarrassed, but they still assert that the stone is Abraham’s. Then we say: ‘Let it be Abraham’s, as you so foolishly say. Then, just because Abraham had relations with a woman on it or tied a camel to it, you are not ashamed to kiss it, yet you blame us for venerating the cross of Christ by which the power of the demons and the deceit of the Devil was destroyed.’ This stone that they talk about is a head of that Aphrodite whom they used to worship and whom they called Khabár. Even to the present day, traces of the carving are visible on it to careful observers.
As has been related, this Mohammed wrote many ridiculous books, to each one of which he set a title. For example, there is the book On Woman,  in which he plainly makes legal provision for taking four wives and, if it be possible, a thousand concubines—as many as one can maintain, besides the four wives. He also made it legal to put away whichever wife one might wish, and, should one so wish, to take to oneself another in the same way. Mohammed had a friend named Zeid. This man had a beautiful wife with whom Mohammed fell in love. Once, when they were sitting together, Mohammed said: ‘Oh, by the way, God has commanded me to take your wife.’ The other answered: ‘You are an apostle. Do as God has told you and take my wife.’ Rather—to tell the story over from the beginning—he said to him: ‘God has given me the command that you put away your wife.’ And he put her away. Then several days later: ‘Now,’ he said, ‘God has commanded me to take her.’ Then, after he had taken her and committed adultery with her, he made this law: ‘Let him who will put away his wife. And if, after having put her away, he should return to her, let another marry her. For it is not lawful to take her unless she have been married by another. Furthermore, if a brother puts away his wife, let his brother marry her, should he so wish.’  In the same book he gives such precepts as this: ‘Work the land which God hath given thee and beautify it. And do this, and do it in such a manner” —not to repeat all the obscene things that he did.
Then there is the book of The Camel of God.  About this camel he says that there was a camel from God and that she drank the whole river and could not pass through two mountains, because there was not room enough. There were people in that place, he says, and they used to drink the water on one day, while the camel would drink it on the next. Moreover, by drinking the water she furnished them with nourishment, because she supplied them with milk instead of water. Then, because these men were evil, they rose up, he says, and killed the camel. However, she had an offspring, a little camel, which, he says, when the mother had been done away with, called upon God and God took it to Himself. Then we say to them: ‘Where did that camel come from?’ And they say that it was from God. Then we say: ‘Was there another camel coupled with this one?’ And they say: ‘No.’ ‘Then how,’ we say, ‘was it begotten? For we see that your camel is without father and without mother and without genealogy, and that the one that begot it suffered evil. Neither is it evident who bred her. And also, this little camel was taken up. So why did not your prophet, with whom, according to what you say, God spoke, find out about the camel—where it grazed, and who got milk by milking it? Or did she possibly, like her mother, meet with evil people and get destroyed? Or did she enter into paradise before you, so that you might have the river of milk that you so foolishly talk about? For you say that you have three rivers flowing in paradise—one of water, one of wine, and one of milk. If your forerunner the camel is outside of paradise, it is obvious that she has dried up from hunger and thirst, or that others have the benefit of her milk—and so your prophet is boasting idly of having conversed with God, because God did not reveal to him the mystery of the camel. But if she is in paradise, she is drinking water still, and you for lack of water will dry up in the midst of the paradise of delight. And if, there being no water, because the camel will have drunk it all up, you thirst for wine from the river of wine that is flowing by, you will become intoxicated from drinking pure wine and collapse under the influence of the strong drink and fall asleep. Then, suffering from a heavy head after sleeping and being sick from the wine, you will miss the pleasures of paradise. How, then, did it not enter into the mind of your prophet that this might happen to you in the paradise of delight? He never had any idea of what the camel is leading to now, yet you did not even ask him, when he held forth to you with his dreams on the subject of the three rivers. We plainly assure you that this wonderful camel of yours has preceded you into the souls of asses, where you, too, like beasts are destined to go. And there is the exterior darkness and everlasting punishment, roaring fire, sleepless worms, and hellish demons.’
Again, in the book of The Table, Mohammed says that the Christ asked God for a table and that it was given Him. For God, he says, said to Him: ‘I have given to thee and thine an incorruptible table.’ 
And again, in the book of The Heifer,  he says some other stupid and ridiculous things, which, because of their great number, I think must be passed over. He made it a law that they be circumcised and the women, too, and he ordered them not to keep the Sabbath and not to be baptized.
And, while he ordered them to eat some of the things forbidden by the Law, he ordered them to abstain from others. He furthermore absolutely forbade the drinking of wine.
99. Cf. Gen. 16.8. Sozomen also says that they were descended from Agar, but called themselves descendants of Sara to hide their servile origin (Ecclesiastical History 6.38, PG 67.1412AB).
100. The Arabic kabirun means ‘great,’ whether in size or in dignity. Herodotus mentions the Arabian cult of the ‘Heavenly Aphrodite’ but says that the Arabs called her Alilat (Herodotus 1.131)
101. This may be the Nestorian monk Bahira (George or Sergius) who met the boy Mohammed at Bostra in Syria and claimed to recognize in him the sign of a prophet.
102. Koran, Sura 112.
103. Sura 19; 4.169.
104. Sura 4.156.
105. Sura 5.Il6tf.
106. The manuscripts do not have the adage, but Lequien suggests this one from Plato.
107. The Ka’ba, called ‘The House of God,’ is supposed to have been built by Abraham with the help of Ismael. It occupies the most sacred spot in the Mosque of Mecca. Incorporated in its wall is the stone here referred to, the famous Black Stone, which is obviously a relic of the idolatry of the pre-Islam Arabs.
108. Gen. 22.6.
109. Koran, Sura 4.
110. Cf. Sura 2225ff.
111. Sura 2.223.
112. Not in the Koran.
113. Sura 5.114,115.
114. Sura 2.
From Writings, by St John of Damascus, The Fathers of the Church, vol. 37 (Washington, DC: Catholic University of America Press, 1958), pp. 153-160.