And you, who were dead in your trespasses and the uncircumcision of your flesh, God made alive together with him, having forgiven us all our trespasses, by canceling the record of debt that stood against us with its legal demands. This he set aside, nailing it to the cross. He disarmed the rulers and authorities and put them to open shame, by triumphing over them in him.

Colossians 2:13-15

The reason the Son of God appeared was to destroy the works of the devil.

1 John 3:8

Now in Christ no accusations can stand against God’s people. “Who shall bring any charge against God’s elect? It is God who justifies” (Romans 8:33). Neither man nor Satan can make a charge stick. The legal case is closed. Christ is our righteousness. Our accuser is disarmed. If he tries to speak in the court of heaven, shame will cover his face. Oh, how bold and free we should be in this world as we seek to serve Christ and love people! There is no condemnation for those who are in Christ. Let us then turn away from the temptations of the devil. His promises are lies, and his power is stripped.

John Piper
The Passion of Jesus Christ

Some question how Satan can be a defeated foe when he appears to be thrashing about so well these days. Well, if you absolutely hate Monty Python, I apologize, but frankly I think this illustrates the situation pretty well... Satan is a looney who still wants to bite our legs off.