Three or four thousand years ago the Jews decided to only worship one god, Yahweh; and some time after that they decided that Yahweh was the only God.
Two thousand years ago, under cultural pressure from Egyptian neighbors, Greek settlers and the Roman occupation, Jews were sliding in their beliefs. Jesus was one of many preachers who tried to restore the people to hardline Judaism through an emphasis on the Shema as the basic prayer: “Hear, O Israel…”
After the Romans crucified Jesus for his Messianic kingship claim, Paul came along. Paul was a Westernized Jew who wanted to spread a religion throughout the Roman Empire, and he decided to do it by blending Jewish monotheism with the polytheist rituals of Rome and Egypt and with the popular military sun-god cult of Mithras. He made Jesus into a Jewish Mithras, elevating him to a divine – or at least mythic – status, and downplaying Jesus’ physical nature.
Fourteen hundred years ago Muhammad, knowing a fair amount about some forms of Christianity as well as Judaism, preached a return to pure monotheism, and the worship of a God who is “One, and eternally sought, who neither gives birth nor is born, and there is none like Him.” Jesus is given recognition as a prophet, but fully human.
So Islam is very close to Judaism and the actual teachings of Jesus, while Paul’s “Christianity” is the odd one out. Think about it: God cannot be human; God can’t be pictured, and you shouldn’t even try; pray to God, not to any saint or any other human; oh, and don’t eat pork. Judaism and Islam agree on all those, and Christianity disagrees. That’s pretty fundamental. And clearly, Christianity isn’t really monotheism at all.