Already in the first chapter of St Mark's Gospel we are witnessing a dramatic encounter between Christ and the devil. The devil makes his appearance through one of his slaves, a demon who has taken possession of a child of God.
When Jesus, the Savior of sinners and conqueror of evil, approaches this possessed man, the demon cries out in panic and desperation. Jesus silences the demonic and frees the possessed man.
The devil is not a fashionable topic for those of us who live in the post-modern world. Yet, the story of Christ's life and ministry simply cannot be told without referring to the devil.
The Apostle John, in his First Letter (4:8), actually sums up Jesus' mission with the following words: "The reason the Son of God appeared was to destroy the works of the devil."
The Gospel writers carefully distinguish between cases of mere physical sickness and cases of a demonic character (both of which Jesus cures). Jesus frequently refers to the devil in his parables and other teachings, and the devil himself tempts Jesus in the desert and returns again later to engineer Judas' betrayal (cf. John 13:2).
This Gospel theme teaches us an undeniable, if uncomfortable lesson: the devil is real, and he is interested in sabotaging the work of grace.
The devil is a fallen angel, an angel who was created good by God but then rebelled against God and took many of his fellow angels with him in that rebellion. And now they try to convince us, human beings, to rebel against God, too.
This basic spiritual truth is a huge comfort. It helps us make sense of all the unpleasant influences at work in and around us - we are not crazy; we are not failures; we are simply engaged in a spiritual battle.
Here's how the Catechism explains it (#414): "Satan or the devil and the other demons are fallen angels who have freely refused to serve God and his plan. Their choice against God is definitive. They try to associate man in their revolt against God."