Satan (who actually became synonymous with the Christian arch-devil, but who in fact is different from Lucifer and from Beelzebub, Mephistopholes, Ba-al, and the rest) began in the Jewish faith as Shaitan- the angel of accusation. In the ancient Jewish religion, there wasn't a hell, and there weren't demons in the sense that we view them. (instead, all souls went to Sheol after death, which was more like limbo or Hades- pretty neutral.) there were Klipoth, which were like pesky imps and fragments of evil men's souls, but still not full-fledged demons. See, this is interesting:
the jews believed that there was One God, not many, and that every other spirit (many worshiped as gods by other religions) fell under the One God's domain. there was nothing outside of the body of God- he was Everything. if something good happened to you, it was god's grace, but if something bad happened to you, then it was God's wrath for some impiety you engaged in. GOD WAS AS "EVIL" AS HE WAS "GOOD," and was largely an encapsulation of the force of nature and all things. angels were subdivisions of god's nature- delegations of his infinite offices basically, and a part of him. Shaitan was that limb of god responsible for accusation, connected to punishment. but rather than viewing this as "evil," the ancients recognized that it was a necessary part of existence, and therefor righteous, though never desirable lol.
THEN, the church evolved, and decided that god was purely "GOOD." but this presented a problem- if you define "GOOD" as that which is pleasurable and desirable to mankind, then how could a perfectly "GOOD" god allow for suffering or cause tragedy? a scape-goat was needed. so the early church took every "negative" angel of the jews, and balled them up into one arch-devil of the embodiment of all "EVIL," that is- everything man doesn't like. this character was biopsied from God and set in opposition to God as its polar opposite. ESSENTIALLY therefor, christianity has not one god, but TWO. its kinda ludicrous to polarize all of existence and set them in opposition to each other, then declare the whole schizo mess a "monotheism"..... and THEN they declare that the "GOOD" side is stronger than the "EVIL", but if there were such an imbalance in nature, the one would quickly cancel out and overcome the other, leaving only the excess of the greater force. so the whole thing is pretty stupid lol.
and THEN in the middle ages, as christianity grew in power and sought to convert the pagans all around them, they took the old gods of the surrounding religions and bastardized and villanized them through propaganda, balling them all up with their "Devil" concept. hence the horns (the celtic horned god, Pan, etc), and all the rest.
long story short- there technically is no Devil in the christian conception, but as with so much else that man molds fabricates from what already exists, enough belief in it will empower it for YOU.