Who is Satan?
http://www.carm.org/questions/whois_satan.htm

The word 'satan' is the Hebrew transliteration of the word 'adversary.' Therefore, Satan is the one who opposes God. Satan, is the most powerful of the fallen angels. Satan, also known as Lucifer and the Devil, is said to be the one who entered the Garden of Eden and convinced Eve to disobey God. There is debate among scholars regarding the time of the fall of Satan. Some say that he fell when he moved Eve to disobey God. Others state that Satan fell sometime before that and merely manifested his adversarial nature in the Garden. Still others say that there was a great battle in heaven long before the Garden of Eden and that Satan took one third of the angels with him.

"And another sign appeared in heaven: and behold, a great red dragon having seven heads and ten horns, and on his heads were seven diadems.4And his tail swept away a third of the stars of heaven, and threw them to the earth . . . " (Rev. 12:3-4).

Of course, the first mention of the devil is found in the Garden of Eden, but it isn't until the post-exilic period (538 B.C.) that the figure of Satan is more prominently displayed: Job 1-2 Zech. 3:1-2; and 1 Chron. 21:1.(1)

Satan is mentioned in the gospels many times:

* Matt. 4:1, "Then Jesus was led up by the Spirit into the wilderness to be tempted by the devil."
* Mark 3:23, "And He called them to Himself and began speaking to them in parables, “How can Satan cast out Satan? 24 'And if a kingdom is divided against itself, that kingdom cannot stand.'"
* John 13:2, "And during supper, the devil having already put into the heart of Judas Iscariot, the son of Simon, to betray Him."

The motive for Satan's rebellion may very well be found in the following verses.

* "How you have fallen from heaven, O star of the morning, son of the dawn! You have been cut down to the earth, You who have weakened the nations! 13"But you said in your heart, ‘I will ascend to heaven; I will raise my throne above the stars of God, And I will sit on the mount of assembly In the recesses of the north. 14‘I will ascend above the heights of the clouds; I will make myself like the Most High.’" (Isaiah 14:12-14).

In the future, there will be a judgment where the devil will be cast out :

* "And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him" (Rev. 12:9).

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References
1. Achtemeier, Paul J., Th.D., Harper’s Bible Dictionary, (San Francisco: Harper and Row, Publishers, Inc.) 1985.