The Bible speaks about spiritual realities and a spiritual world that exists beyond our sight. In that unseen world-which interacts all the time with the visible world-God is at work to accomplish his plans and purposes for His creation. But that world also contains evil forces, headed by a being called Satan, who leads a host of demons that constantly work against God and the people of God.
One of the reasons Jesus came into the world and died on the cross was to defeat the power of Satan and to conquer death. Because He did this-rising from the dead-He now has the upper hand and His power is far greater than that of Satan, who will one day be totally defeated and cast into what the book of Revelation calls the lake of fire.
The Bible considers every person who is not a follower of Jesus to be in the kingdom of Satan-sometimes called the kingdom of darkness. The gospel of Jesus Christ is a call for people to leave that realm and, by becoming a follower of Jesus, become part of the Kingdom of God. Those who do not will perish with Satan, but those who turn to Jesus in faith will live forever with Him.