So define what it is to “BE” a Christian.
That’s a lot harder than what I did. People have had protracted wars over the definition of what it means to be a Christian. And those wars, I might add, in no way settled the issue.
But it is considerably easier to point out beliefs that are not Christian, or even Judeo-Christian. For example, Judeo-Christian religions, including Islam, are monotheistic. A unified God or a unified Trinity God, it still amounts to the same thing, a single God.
A problem arises with the concept of Satan, because it is an easy, wrong, extrapolation to imagine Satan as an “anti-God”, that is separate, opposite and equal to God. Theologians will slap this wrong idea down, and hard, but it leaves us with a dilemma.
There is God, and there is man. But the existence of Satan implies that there is something in between the two, in terms of power. And unlike a monotheistic God, there is no limit to how many entities can exist that are more powerful than man.
But if they are more powerful than us, what sense does it make for us to try and define what they are?
The Catholics tried, creating a very complicated idea of the heavenly host of many orders of angels and other beings. But they likewise fell into the trap of elevating Satan and his minions, who in Hell tried to recreate the appearance of heaven in Hell, as in castles, banquets, and hierarchies.
But this ends up looking silly. Because it is not up to humans to define what it means to be angels or demons. They are beyond us.