This imposes a rather interesting question and realization: it is up to us to discern good and evil, right and wrong, God and Devil. A Devil will just as much tell us to follow him as if he were God as would God himself. So how do we tell them apart? I obviously cannot simply point to any one person's statements as truth on this matter since the Devils themselves could easily have written their own criteria. How would we know? Still, I think the words attributed to Jesus ring a bell of truth in that we can distinguish by the fruits (or actions or outcomes) of those who make proclamations. We have it built within us to distinguish good and bad and we should use that to determine who is God or Devil. What else could possibly be done?
Now I know what some people are thinking which is that we should be comparing with "the Word of God." But this is circular reasoning. If you already believe it to be the Word of God, you cannot use it to distinguish itself. You've already bought in. We are attempting to start from a position of knowing nothing ahead of time. If someone has never heard of your particular religious text and there are 20 books from a multitude of religions before them, how does that person distinguish right and wrong? How do they know which book or books come from God and which come from the Devil? They cannot pick a book at random and claim they know it's of God because it says it is. They cannot arbitrarily say with certainty that claims must be filtered through any one of the books to determine statements to be from God. The only thing that person can do is make a judgement call via their own understanding of right and wrong or else some fallible human "feeling" of which is right. Even if a supernatural presence manifested itself at that time and called out one particular book, how would that person know that God himself manifested and not one of the deceiving Devils? Again, it comes down to that person being required to discern between them. To know and recognize good and evil from the actions and outcomes of ideas, persons, and beliefs.
In short, without already believing in the Bible, for example, one could not filter good or bad through it. It has to first be determined through its fruits to be good or bad. Now, if you "know" the book to be God's Word, then you will likely determine ahead of time that it must meet all the criteria of good and you will be far more likely to make excuses for how it might possibly fit goodness even for things that might appear bad. Similarly, followers of the Quran will do the same along with any other subjects of any other multitudes of religions. So the goal here is not to identify and conclude, but rather to build a criteria for analysis and to encourage the ability and requirement for each of us to think for ourselves lest we be led astray by those Devils wishing to deceive us.
Given that we have no possible conclusion to make but that we should use our own minds and judgements to discern good and evil, I would dare say that one of the first causes a demon might take up is to silence such discerning and questioning since doing so might reveal who they are. A Devil would thus say that one must believe them unquestioningly and unwaveringly to prevent such thought. But why would willingly stop thinking or questioning? It only makes sense if there are dire consequences for questioning, if it were perceived as an insult to the extreme, and if one already believes that person to be God. So if God himself comes down to talk to you, and you "know" it is God and you question "are you sure?" it seems reasonable for God to respond, "Are you seriously questioning me? What is wrong with you??" I mean, this is God, after all. But if you take away the knowledge, if you are not sure if this is God speaking to you and you are trying to discern it, how does one feel about those statements now? Imagine it was just me or some magician telling you do do something that you found questionable and they responded with incredulity that you'd dare question them. Would it convince you? Of course not. You'd find that person all the more off-putting that he'd attempt to take away your ability to reason and expect compliance. It is a Devil's tactic. Scare you into submission and turn your brain off from considering if he is actually the Devil in disguise.
So to recap, a Devil would say to believe him, follow him, don't question him, don't think for yourself, and would threaten you severely for if you do otherwise. Interesting. And once this Devil has you in this position, you are a slave to his desires. If he tells you to kill someone, you can't question it. You wouldn't dare. Severe consequences! And this is God! How dare you question! He must have a great reason to kill this person. Don't think, just do. And who would he have you kill? Someone who tries to out him as the Devil, of course. Someone who uplifts thinking and is a danger to his reign and power. A "blasphemer" if you will.
This all seems kind of obvious. Under what circumstances would a benevolent being operate under these conditions? Benevolence would do the exact opposite. A being of goodness would uplift you and not simply insult you. Think about our own day to day interactions with humans. If all your boss did was tell you that you were trash and shouldn't question him or else get fired, it should be clear in a nanosecond that this is a bad boss. A good boss would tell you that you are capable, strong, smart, and grow you to greater potential. A bad boss would fear you taking over his position while a good boss would take pride in your accomplishments. The exact same thing goes for parent and child relationships. A Devil will fear you, put you down, silence you, say you are worthless, and withhold knowledge and wisdom to retain power. A Devil would make you fully dependent upon him and force you to kill all those who oppose him or uplift a genuine thought of consideration.
Perhaps you already see where I am going with this. The largest religions we have seem to be entirely centered around these exact principles. You are nothing without "God"--naught but filthy rags even. You must believe or burn in hell. Don't you dare question or blaspheme. You must kill the non-believers or infidels. How many wars did "God" supposedly require in the Bible or the Quran entirely unprovoked? How many wars and mass genocides did leaders of churches demand? What one calls God is not indeed the God of goodness and righteousness if it is centered around the very behaviors and demands of a Devil. This is the Devil himself who proclaims himself to be God while calling anyone "Satan" who may dare to question him. Such a God is a wolf in sheep's clothing and not even good clothing at that. Such a "God" gets away with it in the same manner as a drug lord. He silences those who pose a risk, discourages independent thought, and scares the ever loving crap out of everyone to prevent them questioning or inspiring any others to question. He encourages outing those who would dare question and killing such competition. And with strength in numbers, he amasses an army of people who will do this exact bidding believing themselves to follow God while being entirely sold into slavery of the Devil himself.
So who in the Bible is God and who is the Devil? It sounds to me that their names are often reversed. Satan said to eat of the tree of knowledge of good and evil? Is that really a bad thing? Perhaps that was God himself trying to free the minds of Adam and Eve who were under the oppression of the Devil himself. The Devil wanted to ensure they could not think for themselves, to follow blindly, and threatened them with death if they disobeyed. The Devil called himself God while shunning and demonizing the true God of love and enlightenment. Or perhaps just some do-gooder named Satan who was given a bad rap. Was it "God" who killed all the first born of the Pharaoh? He didn't even kill Cain, a legit murderer, but he's going to mass murder children? This does not sound like God but a Devil. Did "God" plague Job just to win a bet with the "Devil?" Of course not. That is not something benevolence would do. These questions and ideas, for many people, will raise fear and possibly even anger or hatred. Does that sound good? Or does that sound like the beginning of evil and a desire to silence one who questions and thinks? Would a true God of love and mercy have those feelings or would a Devil of anger, malice, and fear of being discovered have those feelings?
Just as Jesus is attributed for saying in the Bible, the wheat and weeds grew together. There are some fantastic things in the Bible, but there are also some horrific ones. The names of God and the Devil are quite often reversed which you can tell by their fruits, demands, and desires. Better than "What Would Jesus Do," consider "What Would a Devil Do?" Not to imitate the Devil, of course, but to be aware of his schemes and the tools with which he controls. Would a benevolent God do this and demand you stop questioning or would a Devil? The name "Satan" by which we often refer to the Devil might very well be a well-meaning person that the Devil himself demeaned to disguise his tracks. How does one know apart from the fruits of the beings? Who demanded blind adherence and who uplifted knowledge? If the mere thought of calling Satan benevolent raises extreme fear and suspicion, you might just have to ask yourself why. Who or what made us fear this name so strongly? Does fear come from the graces of a benevolent loving God or does fear come from a malevolent Devil bent on power and permanent authoritarian rule with the forced admiration and continual praises of his enslaved constituents under threat of hellfire? It really begs the question. And if you fear to question, that just begs the question all the more. Who do you really serve?
In the end, as Jesus is yet again attributed for saying in a multitude of locations (and which makes total sense) the true God of love and peace will judge your actions and behaviors--not whether or not you simply claimed to have served "God" as proclaimed by a Devil. A benevolent God would encourage a well-reasoned thought process and desire for goodness and accuracy while an evil Demon would demand blind faith, legalism, dogma, and your unquestioning faith and admiration under fear of eternal torment.
All of the above may suppose that God(s) or Devil(s) exist, but even supposing otherwise we come to the same conclusions: we must reason right from wrong and choose which we will serve. It seems to me that Jesus meant to reinstantiate this understanding. For that, he was killed, and the Devil once again took over the narrative as any decently capable Devil would do.