St. John of Damascus lived in Damascus and held high office under the Muslim Caliph in the 8th century, and he regarded Islam as a Christian heresy, not as a separate religion. That was certainly the view of the Christian king of Ethiopia, with whom the early Muslims found refuge, and I agree. I think it's clear that the animosity, which has always been far more prevalent on the Christian side, is a Pepsi-Coke thing. It's the bitter rivalry of those that are alike. Christians have historically hated Muslims and Jews far more than Hindus or Buddhists, for instance, because they are most like us.
Muslims, as I first found in the Muqaddima, believe that Jesus Christ will judge the world on the last day. He is called the Spirit from God and also the Word of God. He rose from the dead.
Historically, I think it's pretty obvious that Islam may be best understood as the eastern Mediterranean version of the Reformation that took place later in Western Europe. These two took different paths for various reasons - for instance, the Catholics didn't collapse in the face of Reformers as the Byzantine empire did before Islam, so that the stalemate of the religious wars in Europe led eventually to both sides losing credibility and giving ground to secularism. That attrition didn't happen in the east, because the Muslims conquered quickly and easily and didn't persecute the Christians. Indeed the Christians were glad to see them, because they were tired of being persecuted by the Byzantines over subtle differences in Christian doctrine, much as a secular Croatian state looks a lot better to Eastern Orthodox Christians than the Roman Catholic Ustashe regime that killed 250,000 of them during World War 2.
Just as the Roman Catholics and the Protestants caricatured each other's doctrines and became more extreme themselves in reaction, the same happened with Islam.
For instance, Islam is horrified by the idea that God might have a son, because the notion of God having sex with a woman and begetting a child on her is an abhorrent notion fit only for the idols of Greek mythology - and it looks like that's where Christians got that idea. Christian doctrine agrees with Islam on that, but the notion of the perpetual virginity of Mary, which has no biblical support, is frequently argued from the premise that Mary became God's wife. This false argument clearly provoked the Muslim response. God does beget sons, but it's not a sexual thing in any way. It is the the divine nature engendered in people who hear and believe God's word, who begot Jesus in Mary through the word of faith that she received from the angel.
That the word of the one God which brought into being the worlds and raises the dead can engender God's nature in those who hear God obediently is not so foreign to at least some variations of Islam. Arabic actually makes all this much more clear than English does, in that walad refers to a son born of sexual union, whereas ibn (related to Hebrew ben) need not. Ibn falistin (son of Palestine), or ibn California, do not suggest that a man was sexually begotten by Palestine or California.
Some Muslims reject the notion that Jesus was actually killed on the cross, based on Surah 4:157, which says, "But they killed him not, nor crucified him." However, the Qu'ran states elsewhere that he certainly did die and was raised, and in fact the mystery is quite easily resolved when we consider what Jesus said himself. "For this reason the Father loves Me, because I lay down My life so that I may take it again. No one has taken it away from Me, but I lay it down on My own initiative. I have authority to lay it down, and I have authority to take it up again" ( John 10:17-18).
We have a similar history of such controversies in Western European Christianity, for instance the controversy over salvation by faith alone, or the nature of the eucharist. It's evident when we look at the scriptures that both sides are right in different ways, which become obvious if we actually examine the scriptures in context. These are what Paul calls stupid controversies, so it should not surprise us that similar stupid controversies arose in the east, as such things do when the parties are aiming to prove themselves right and the other wrong, rather than to learn the truth.
The present Muslim-hating rage in the United States is quite obviously a manifestation of the kind of pre-genocidal hatred that led to Russian pogroms and the Nazi annihilation of the Jews. It is not surprising in a nation with a Crusader mentality, that wants to bring its gospel to the world with bomb and missile. But it remains shocking that people who claim to be disciples of Jesus should have anything to do with it, and still worse that they should even take the lead in such wickedness.
And since it's no new thing, but the kind of hatred and love of persecuting that we've seen in professing Christians against the Jews and others for the past 1800 years or so, we need to consider that it isn't just Muslims or Jews that are radically mistaken about some things, but Christians too. Jesus said that the tree is known by its fruit. When we look at the fruit these past centuries, considering the record of murder, persecution, robbery, and genocide that Christians have regularly participated in, don't we need to go to Doctor Jesus and get thoroughly and correctly diagnosed and cured before we have anything to say to Jews or Muslims?
The proverb says, "Who can say, 'my doctrine is pure; I am free from my sin?'" That's pretty clear. The way we know our doctrine is pure is when we're free from sin, so if we're not free from sin, something is wrong with our doctrine. The Bible knows nothing of right doctrine and wrong behavior. "The tree is known by its fruit." Our wrong behavior is God's temperature gauge warning us that we have him wrong somehow. Our emphasis on bullet items like the Trinity, baptism, and all the rest - even though it's good to get these right - is our way to kid ourselves that we don't need to heed that gauge. But Jesus says we do.