It’s interesting that Christianity is the only mainstream religion that has a clear concept of a Devil who is an adversary of God and who competes with Him for the souls of human beings. The only other religion that I know of which has a similar concept is Buddhism, which (au contraire) does not have a God, but instead has a devil in the form of Mara, the tempter, who seeks to distract the devout. But even Mara is accepted widely as being only allegorical, the battle with him being an internal battle with temptation. Only in Christianity does the Devil interlope on God’s creation, surreptitiously entering the Garden of Eden, tempting Eve with an apple, and then seeks continuously to cast mankind into hell.
However, in both Judaism and Islam, there is the concept of an “adversary”. In Islam, this adversary is known as Shaitan and he tempts human beings. But Shaitan’s role is not to rule his own little kingdom in Hell in competition with God, but rather to serve as an instrument of God in ensuring that human beings are able to separate good from evil, doing good even if they are tempted otherwise. God does not war with Lucifer (as he does in Christianity) and cast the fallen angel to Hell from where he rules; in Islam, God creates Shaitan as well as everything else in the world, and evil is as much a part of God’s world as goodness is.
In Judaism, the concept is even more nebulous. In the book of Job, for example (my favorite book in the Bible), there is God, and there is an ‘adversary’ who needles God into torturing Job to prove Job’s faith. This “adversary”, though certainly not ‘good’ per se, is still not cast as God’s enemy; rather, he is cast in the light of some kind of annoying neighbor who drops into God’s parlour for a cup of tea and a bottle of provocation. Some people (myself included) also believe that the God and the Adversary in the Prologue of Job are very different from the GOD who appears in the whirlwind and speaks to Job at the end of the book. After all, how could the little god of the Prologue justify himself to Job? — when Job, at the pit of this morale, raises his hands to heaven and asks God, “Why, God? I have always been pious and faithful; why this suffering?”, the little god of the Prologue cannot very well reply, “Well, Job, you see, I made this little bet with the Devil, and…”
Hinduism, of course, has no Devil. It has a very complex God, and no devil at all. It does have the concept of the Devas and the Asuras, but these demi-gods and demi-demons populate a plane that is only one step up from that of us humans; neither of them lay claim to anything more permanent or more creative. They are the equivalents of the djinns of Islam, with no serious effect on Hindu theology.
Interestingly, I always thought Zoroastrianism had a Devil; I remember reading (this was in Carl Sagan’s Contact, if I am not mistaken) that in a piquant inversion, the word Ahura (from Ahura Mazda, the supreme Godhead of the Parsis) is the root of the word asura (demon in Hindu mythology), while the word devi and Devil are both derived from the name of the embodiment of evil in Zoroastrianism. Wikipedia tells me that the devil does have a name, Angra Mainyu, and that Ahura Mazda himself is a rather peculiar sort of God, because while he is omniscient, he is not omnipotent. So while the Parsis hope that good will eventually triumph over evil, they are not guaranteed that this will happen. So Ahura Mazda is not exactly God, and Angra Mainyu is not exactly the Devil.
So that brings us back to the original interesting discovery of the day: that while all religions have a God, Christianity is the only one that has a Devil.