The devils are fallen angels, enemies of mankind. Their efforts are directed toward leading people into their own rebellion against God.
God has not made known to us the test to which the angels were put prior to the fall of the devils. Some Church Fathers and theologians have speculated that God the Father gave the angels a preview of Jesus Christ, the Redeemer of the human race, and commanded that they adore Him. They, perhaps, could not bring themselves to make an act of adoration toward Jesus Christ, and so they rebelled against the Father’s plan of Salvation.
Led by one of the most gifted of all the angels, Satan (or Lucifer, that is, Lightbearer), many of the angels turned away from God. Satan cried out, “I will not serve!” Then the Archangel Michael took up the battle cry: “Who is like God?” Michael and the good angels cast the rebellious angels out from Heaven. “Now war arose in heaven, Michael and his angels fighting against the dragon; and the dragon and his angels fought, but they were defeated and there was no longer any place for them in heaven. And the great dragon was thrown down, that ancient serpent, who is called the Devil and Satan, the deceiver of the whole world—he was thrown down to the earth, and his angels were thrown down with him” (Revelation 12:7-9).