The concept of spiritual warfare is based on the understanding that Satan and demons are real beings whose primary mission is to thwart God's purposes on earth, specifically to prevent non-believers from placing faith in Christ and to prevent Christians from being effective disciples of Jesus Christ.
The Scriptural basis of spritual warfare is found in the Apostle Paul's Letter to the Ephesians.
A popular modern portrayal of spiritual warfare is Frank Peretti's book, This Present Darkness.