Question: "Who/what is Satan? Who/what is the devil?"
Answer: People's beliefs concerning Satan range from the silly to the abstract - from a little red guy with horns who sits on your shoulder urging you to sin, to an expression used to describe the personification of evil. The Bible, however, gives us a clear portrait of who Satan is and how he affects our lives. Put simply, the Bible defines Satan as an angelic being who fell from his position in heaven due to sin and is now completely opposed to God, doing all in his power to thwart God's purposes.
Satan was created as a holy angel. Isaiah 14:12 possibly gives Satan’s pre-fall name as Lucifer. Ezekiel 28:12-14 describes Satan as having been created a cherub, apparently the highest created angel. He became arrogant in his beauty and status and decided he wanted to sit on a throne above that of God (Isaiah 14:13-14; Ezekiel 28:15; 1 Timothy 3:6). Satan’s pride led to his fall. Notice the many “I will” statements in Isaiah 14:12-15. Because of his sin, God permanently removed Satan from his exalted position and role.
Satan became the ruler of this world and the prince of the power of the air (John 12:31; 2 Corinthians 4:4; Ephesians 2:2). He is an accuser (Revelation 12:10), a tempter (Matthew 4:3; 1 Thessalonians 3:5), and a deceiver (Genesis 3; 2 Corinthians 4:4; Revelation 20:3). His very name means “adversary” or “one who opposes.” Another of his titles, the devil, means “slanderer.”
Even though he was cast out of heaven, he still seeks to elevate his throne above God. He counterfeits all that God does, hoping to gain the worship of the world and encourage opposition to God's kingdom. Satan is the ultimate source behind every false cult and world religion. Satan will do anything and everything in his power to oppose God and those who follow God. However, Satan’s destiny is sealed - an eternity in the lake of fire (Revelation 20:10).