The development of the concept of the Devil in the history of religion is fascinating to ponder. Originally, Israelite tradition did not speak of a devil per se, but only of being tempted to do evil or to worship gods other than Yahweh. After the Book of Job—probably as a result of exposure to Zoroastrianism during the Babylonian Exile—the Jews began to develop the concept of Satan, not only as an adversary/prosecutor of humans, but as an opponent to God as well. Christian tradition expanded this, developing the concept of Satan as the fallen angel Lucifer who rebelled against God. Islam continued this tradition, but adopted the earlier Hebrew view that Iblis (Satan) can do nothing outside of the will of Allah.
Unificationism shares the Christian view that the Devil indeed was once the archangel Lucifer. It shares with Islam the insight that Lucifer's motivation was jealousy, because he had been higher than Adam and Eve prior to their creation. Reverend Sun Myung Moon teaches that the Devil gained power over humankind through the sin of adultery, which he committed with Eve. This agrees with the rabbinic opinion that the Devil was the paramour of Eve, although it emphasizes that this was a spiritual act, not a physical one. Thus, Satan is not the actual father of Cain, but is the spiritual father of all humankind.
The Messiah comes to restore what was lost in the Garden of Eden. He is thus the second Adam, whose mission is to restore a second Eve and establish a sinless lineage into which all humankind will eventually be adopted. Knowing this, the Devil is indeed the arch-enemy of the Messiah, and of each human being. However, once people know the formerly secret truth of Satan's crime and the manner in which he came to rule—through his sin of illicit love with Eve—people can turn the tables and become his prosecutor before God, instead of the other way around.