Before God created people, He made multitudes of angels who lived with Him in heaven. The angels were spirits and were created good, without sin. God made them to serve and worship Him. One of the angels created by God was more intelligent and more beautiful than all the others. His name was Lucifer. God gave Lucifer an important position in heaven.
At some point Lucifer became very proud and wanted to become as important as God and to take God's place. Many of the other angels also rebelled against God. But God knew what Lucifer and his followers were thinking. The rebellion failed because God knows everything and is more powerful than Lucifer, who was created by God.
Because of their sin, God cast the sinful angels out of heaven. God is righteous and cannot stay with those who rebel against Him. God also said that one day He would complete his punishment of the sinful angels and cast them into the Lake of Fire.
God changed the name of Lucifer to Satan. The name means "Deceiver". Satan and the bad angels became God's enemies. They oppose the plan of God whenever they can. Now, those angels who followed Satan are the evil spirits here on earth. They follow Satan and rebel against the truth. Satan does not want us to listen to God's Word. But those who trust in God do not need to fear Satan or his demons because God has power over them all.