From what I have heard, Mormons believe that Jesus was a created being and was the brother of Lucifer. Perhaps they believe that Lucifer was rebelling against his brother. After all, good Mormons, sealed in a Mormon Temple "for time and eternity" are supposed to be granted their very own universes, "gods" of their own creations!
Lucifer was one of the three Archangels. He was "anointed as a guardian cherub . . . blameless in [his] ways from the day [he was] created till wickedness was found in [him]." Ezekiel 28:14, 15(NIV). The other two Archangels are Michael and Gabriel. Each commanded a third of the angels. When Lucifer tried to ascend to the throne of the Most High God, he and his third of the angels were cast "down" to the earth. He became Satan, or the devil, and his fallen angels became demons.
Satan is the "Prince of the Power of the Air" and the lord of the earth and the present age. That is why he has so much power here on earth; but Jesus broke his stranglehold by living a sinless life, dying on the cross for the sins of all mankind, and rising again from the dead. Just remember, the sins of everyone on earth today were committed *after* the death and resurrection of Jesus! Yet He died for those sins!
We Christians need not fear Satan's power, as long as we follow this admonition: "Be self-controlled and alert. Your enemy the devil prowls around like a roaring lion looking for someone to devour." I Peter 5:8(NIV). However, we need to daily put on the full armor of God (all seven pieces), as we are admonished to do in Ephesians 6:11-18!
Satan does not fight against non-believers, since he already has them! However, he is constantly fighting against Christians, when they try to tell non-believers the Truth of the Cross!