In the Bible we learn that the devil roamed the earth going to and fro since the old ages (in the Book of Job, which is the oldest book in the Bible, it states this). He is called the accuser and the father of lies. He was called lucifer in Heaven, and was in charge of music. That is why so much music is anti-god as it is satan’s speciality. In the Book of Revelation we are told that lucifer rebelled against God, being puffed up with pride, along with a third of the angels in Heaven, and they were cast down to hell, hence the term “fallen angels”. Satan can invoke thoughts in people minds, and this is not sin, but if you entertain the thought and let it get hold, and it causes you to commit sin, the sin is added to your debt and you have fallen outside of Christ. You need to repent of your sin.
In the Bible we are told that people can become possessed with many demons. Mary Magdalene had seven which were cast out of her. Demon possession causes people to err and sin against God, giving into various lusts and depravity. Satan even entered Peter, and caused Peter to deny Christ three times, and also caused Peter to try to convince Jesus not to go to the cross, and Peter loved Jesus very much. Jesus taught us not to hate the person, but the sin they commit. So here we have learnt that the devil can enter people around you, even those you hold dear, try to set you up in certain situations and demons can possess you and others. To conquer the devil, keep you focus on Christ. Follow the Apostle’s Doctrine (follow Jesus’ teachings, have Holy Communion, have Christian fellowship and pray often).