Muhammad, according to Islamic belief, was given the Qur’an from Allah, through the angel Gabriel. The Qur’an denies central tenets of Christianity in numerous ways:

Jesus is not the Son of God, and belief in the Trinity is “excess”: “O People of the Book! Do not exaggerate in your religion nor utter anything concerning Allah except the truth. The Messiah, Jesus son of Mary, was only a messenger of Allah, and his word which he conveyed to Mary, and a spirit from him. So believe in Allah and his messengers, and do not say “Three.” Cease! Better for you! Allah is only one Allah. Far is it removed from his transcendent majesty that he should have a son. His is all that is in the heavens and all that is in the earth. And Allah is sufficient as defender.” — Qur’an 4:171

And: “It is not befitting to Allah that he should beget a son. Glory be to him! When he determines a matter, he only says to it, ‘Be,’ and it is.” — Qur’an 19:35

Jesus was not crucified: “And because of their saying: ‘We killed the Messiah, Jesus son of Mary, Allah’s messenger’ – they did not kill him nor did they crucify him, but it appeared so to them; and those who disagree concerning it are in doubt about it; they have no knowledge of it except pursuit of a guess; they did not kill him for certain.” — Qur’an 4:157

Those who believe in the divinity of Christ are unbelievers: “They have certainly disbelieved who say that Allah is the Messiah, the son of Mary.” — Qur’an 5:17

Christians have forgotten part of the divine revelations they received: “From those, too, who call themselves Christians, we took a covenant, but they forgot a good part of the message that was sent them: so we estranged them from one another with enmity and hatred until the day of judgment. And soon will Allah show them what it is they have done.” — Qur’an 5:14

Those who believe that Jesus is God’s Son are accursed: “The Jews call Ezra son of Allah, and the Christians call the Messiah the son of Allah. That is a saying from their mouth; they just imitate what the unbelievers of old used to say. Allah’s curse be on them: how they are deluded away from the Truth! ” — Qur’an 9:30

Christians who do not accept Muhammad and the Qur’an are the most vile of created beings: “Nor did those who were given the Book become divided until after there had come to them clear evidence. And they were not commanded anything except to worship Allah, and to be sincere to him in religion, inclining to truth, and to establish prayer and to give zakat. And that is the correct religion. Indeed, those who disbelieved among the People of the Book and the polytheists will be in the fire of hell, abiding eternally in it. Those are the most vile of created beings.” — Qur’an 98:6

Muslims must fight against and subjugate Christians: “Fight those who believe not in Allah nor the last day, nor forbid what has been forbidden forbidden by Allah and his messenger, and do not acknowledge the religion of truth, even if they are of the People of the Book, until they pay the jizya with willing submission, and feel themselves subdued.” — Qur’an 9:29

“Advent-cum-Mawlid: church holds joint birthday celebration for Mohammed and Jesus,” Archbishop Cranmer, December 4, 2017 (thanks to David):