Is devil the sine qua non of God’s existence?
You can’t make a hero without a villain. It is the villain who makes a hero. There will be nothing called good if there is nothing bad. It is like saying if everyone is vaccinated, you don’t need one.
This kind of dichotomy is the fundamental cornerstone of the universe. There is no white if there is no black, no light if no darkness, no heaven if no hell, no virtue if no vice, no “punya” (= good deed – “Mitzvah”) without sin. So in effect, both are complementary to each other. Each depends on the other for its very existence, rather symbiotic. That is the reason why the evil is portrayed to be as powerful, if not more, as the God(s) in all mythologies and religions, ancient or new!
In Christianity, God and the Devil (aka Satan, aka Lucifer) are usually depicted to be perennially fighting to exercise control over the souls of humans, with the Devil seeking to lure people away from God and into Sheol. The Devil commands a force of lesser evil spirits, commonly known as demons.
In Islam the Devil is referred to as Iblis or “Shaitan”. According to the Qur’an, God (called “Allah” in Arabic) created the Satan out of “smokeless fire”, while He created man out of clay. The primary characteristic of the Devil, besides hubris, is that he has no power other than the power to cast evil suggestions into the heart of men and women. Fundamentally Islam holds its believers in a tight leash by constantly harping on the threat of Shaitan’s influence on their lives (as well as afterlife), if you do not conform strictly to the tenets. That way, and through other blandishments like 72 of the choicest and full-breasted virgins soaked in copious flow of wine in heaven, Islam turns even doctors of medicine into human bombs (“Fidayeen” terrorists), ready to sacrifice their precious lives to annihilate “kafirs” meaning non-Muslims!