"Jesus said to him, 'Away from Me, Satan! For it is written: 'Worship the Lord your God, and serve Him only." Then the devil left Him, and angels came and attended Him'" (Matthew 4:10-11).
The terms Satan and the devil are mentioned over eighty times in the Bible. Each time it is in reference to a personal being. Satan literally means adversary.
The popular modern portrayal of Satan is that of a red-costumed comic character who parades around forcing people to do things against their will; or who presides over a place called Hell, where he and his demons gleefully torment the residents. These caricatures have no foundation in Scripture.
Many Christians believe Satan and Lucifer are one and the same being. Other Christians see them as two unique beings. He is a powerful, created being who abused his power and status in an effort to overthrow his Creator: "You said in your heart ... I will make myself like the Most High" (Isaiah 14:13-14). This should not be a difficult concept to accept since much of humanity also rejects the Creator, preferring instead to be their own "god."
Scripture portrays Satan as having the desire and ability to influence human beings to perform works of evil: "Your enemy the devil prowls around like a roaring lion looking for someone to devour. Resist him" (1 Peter 5:8-9). This too should not be a difficult concept to accept -- human beings often intentionally influence others to perform evil actions, so it is not unreasonable to accept that beings in the supernatural realm may have a similar desire and ability.
Satan cannot force Christians to do things against their will -- he can tempt, but he cannot force. The only effective Christian response to Satan's temptations is to use the word of God. The Lord himself demonstrated this in Matthew 4, when, each time Jesus was tempted, He quoted Scripture.
The secular world, and many cults, deny the existence of Satan. They do so, not for lack of evidence. Historic Christianity affirms his existence, as well as his eventual defeat: "And the devil, who had deceived them, was thrown into the lake of burning sulfur, where the beast and the false prophet had been thrown. They will be tormented day and night for ever and ever" (Revelation 20:10).