Points of disagreement
So what do the “Abrahamic” religions disagree about?
Among other things: the purpose of humanity, the relationship of God and humanity, sin, forgiveness, salvation, the afterlife, Jesus, Muhammad, the calendar, and the religious importance of Abraham himself.
Plus the nature of God.
Any summary will leave out enormous nuance. Internal divisions within religions have fueled some of the worst examples of human violence. Consider the long and frequently bloody history of troubles between Catholics and Protestants or the growing death toll of Muslim-on-Muslim attacks.
But there are common elements about God widely accepted in each tradition.
Related: Debating why millenials are leaving the church
Start with Judaism, since it came first and established roots that carried into the other two.
Jewish tradition teaches that there is one and only one God, creator of everything, and He established physical and moral laws. As Judaism’s preeminent prayer says: “The Lord our god, the Lord is one.”
This God walks and talks directly with His creations – for a while.
Eventually, He chooses one particular nomad (Abraham) to father a mighty nation that God sets up as an example to other nations.
This God likes the smell of burning meat and demands other extremely specific physical offerings as evidence of obedience and repentance. And He gives His chosen people a particular set of laws – but doesn’t mind discussion and even argument about those laws.
A famous rabbinic saying implies that every word in Judaism’s sacred texts can be understood in 70 correct (but related) ways. And human reasoning can even trump divine intention. (No kidding. It’s in the Talmud)
This God judges His people every year. Tradition says he’s willing to accept imperfection, as long as it comes with repentance.
He’s big on obedience, not so much on faith. He’s not nearly as attentive to the behaviors of non-Jews. (There’s a famous Jewish joke with the punch line “Would you mind choosing somebody else once in a while?”)
Tradition holds that there’s a World To Come after death where moral accounts will somehow be settled. But this God is vague on details.
The most obvious differences in the Christian God are the traditional teachings about the Trinity and Jesus. God is three separate persons who are also one. How? Christianity says the Trinity is a “mystery” of faith.
According to Christian tradition, God begets a son who is somehow also Him but not Him to atone for Original Sin. He sacrifices that son though a brutal death and thus achieves humanity's salvation.
But the son, who also is God, rises from the dead. And that sacrifice redeems eternally all who accept and believe in it. Faith, not behavior, is the essential measure of salvation.
This God is willing to vastly expand what it means to be among His “chosen people.” He’s also willing to cancel many of the laws that had applied to that chosen group for this expanded membership.
Orthodox Jews say that God prohibits them from eating a cheeseburger; Christians say God has no problem with them wolfing down Big Macs.
Unlike the Jewish God, whose instructions are almost all about this world, the Christian God is focused more on eternal salvation: heaven and hell.
Finally, for this God, much of the Jewish scriptures (which are all God’s word) are actually about foreshadowing Jesus. Including Abraham.