Also, would we be without sin if the Devil didn't exist?
The immaterial version of the Devil would just amount to the global principle of an Adversary at work in a created realm. Few authors would want a dull story, especially if it's meant to be entertaining rather than a technical manual. So a type of trouble-making has to be introduced (whether the latter involves sin specifically or not). Since the product addressed here is a world which continually begs for causes, reasons, or explanations for any trend, object, circumstance, or event... Then accordingly that would also require a source of responsibility for triggering the trouble-making. It doesn't necessarily have to be an adversary or a single agency behind it all from beginning to end; but if the saga or the folklore features a protagonist, then an adversary is likely to complement such.
In a novel, the protagonist may defeat or kill its adversary at the end of the story. But all the incidents featured in that author's account still co-exist with each other. The adversary still remains in the the earlier sections of the book. Despite having a point where it is disposed of, the adversary is still a general principle or regulating idea pervading most of the story; and is locally realized as a particular (body) in those events involving it (or so the printed text conjures such in the imagination of the reader).
If the tribal belief is that God is the top idea that all the others are subsumed under, then to be internally consistent God couldn't be subservient to concepts like causality and change. (That's "change" it terms of one state of _X_ taking its temporary turn to exist rather than co-existing permanently with the other states). So eternalism is more the bed that God would be residing in rather than presentism, though technically God would also have to be dominant over even that view of time (i.e., God would be prior in rank to any environmental condition or relational context). So more along the line of the angels and demons (including Satan) being indestructible denizens of an eternalism level.
Even if an individual manifestation of the Devil as a body was slain, he or his influences would still be realized as something concrete in other locations of time / space. The principle of the Devil would pervade and contaminate the framework (or to put another way, specific regions of the framework conform to the dictates of the adversary principle).
Of course, the same might apply to humans as well (that they're embedded in some kind of block-universe, or multiverse version of that if subjectively contingent "free will" is argued by the tribal canon). But people would be generative principles with lesser range than those of the celestial beings. Like Wilma Smith only instantiated as a body living from 1920 to 1962, rather than sporadically popping-up all over the place throughout history as either a blatant corporeal-like phenomenon or just cryptically concealed effects.
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