As counterproductive as it may seem, opposition can be a powerful motivator. Whether the race of humans was created for the sake of becoming slaves to work the earth—such as in the Sumerian tradition—or slaves to a deity, as the Hebrew model puts forth—it has a task. Any job needs sufficient motivation to get done, and as good as the promise of divine reward may seem, sometimes it cannot be good enough, as it is capable of being withheld or is promised in another time.
Even divine punishment can be withheld for many reasons, Divine mercy being probably the most well-known among modern readers. It cannot be a consistent punisher or a consistent motivator. As noted above, if the race of humans needed something to get its own job done, it would be an entity to call “other”—one outside of humanity and outside of the worshipped.
The monotheistic religions fulfill this requirement with the advent of Satan. Genesis has a primeval figure before Satan is mentioned anywhere—the Serpent in the Garden of Eden. He causes not only humanity to stumble, but he is demoted and put to shame. The force the serpent exerted in his form on earth remained in heaven as the Devil, who remained in power after the Serpent was, paradoxically, weakened by sin.
But G-d does not really need Satan. Humanity needs Satan. Why is the Devil necessary? For the sake of doing good, humanity’s hate must be controlled or otherwise directed somewhere. Despite the fact that in a monotheistic religion, G-d controls everything, an evil messenger in the form of Satan can contain the hatred that would otherwise be directed at the Supreme G-d.
For example, in the opera “Tales of Hoffman” the poet Hoffman blames the Devil for his shortcomings instead of displaying resentment towards G-d. But ultimately, the Devil says that he has little control in the situation—which is above all a fair assessment. “He makes me lose at cards”, the poet says, to which the Devil replies “That’s because you don’t know how to play!”
Satan does have power as a suggesting force—an advisor that wants to see things come to harm, which is illustrated in the Book of Job as he sways even G-d to his whim. Like the djinni and the efreets in “The Thousand and One Nights”, which largely play the same role as Satan and are translated by Husayn Haddawy as “demons”, Satan uses a system of justice and rationalization in order to cause malevolent deeds to come forth.
Unlike them, he has no true power to actively kill someone—although Satan himself hints at having this power in many stories throughout different traditions and in popular culture. The inability to kill is highlighted in some religions by an equation of the desire to commit evil and the Devil. The hate of humanity, which is directed at the Devil, is also directed at the source of all hate—which is the Evil Inclination and the same as Satan. By means of the Devil, hatred is used to weaken hatred. It is a weapon turned on itself.
The true salvation from the Devil does not come from Heaven, but instead from humans. In the Jewish tradition, for example, it is the Messiah—and not G-d—that puts forth Satan to utter shame. With enough love among humanity, we will need the Devil no longer, as he thrives on an audience, and without the audience he will old no power over anything.