It would have been common knowledge in the church that the devil was the enemy of their souls. What does John want them to see from this unusual vision of their enemy? First, John would want them to remember that although the devil is only a creature he does possess unusual abilities (seven heads, with eyes that can see in lots of directions, and by extension, ears that can hear a lot, and tongues that can say a lot), power (ten horns – he has an empire), authority (diadems point to rule) and followers (a third of the stars may refer to fallen angels). Something of his ability and power is seen in the way he was described as forming a river to engulf the church.
Second, John wants them to remember that the devil attacked Jesus when he was born, but those attacks were unsuccessful. We can think of the attempt of Herod to kill the infant. It may be that John wants his readers to think about the period between the birth of Jesus and his exaltation and recall how unsuccessful the attacks of the devil had been on the Saviour throughout that time. Several are mentioned in the Gospels. It is obvious from them that the powers of darkness knew that Jesus was the Lord and that their fate was decided.
Third, John mentions that the devil was thrown out of heaven after a rebellion that was put down by the archangel Michael and loyal angels. When did this happen? The heavenly announcement seems to connect it to when Jesus ascended to the throne. Jesus, in John 12:31, a verse spoken in connection to his death, says that would be the time when Satan would be cast out. Probably he resisted in some way and discovered that his power was ineffective in comparison to the divine power that enabled Michael and his army.
In Old Testament times, the devil was given access to heaven to accuse Job falsely. Joshua, the high priest with defiled garments mentioned in Zechariah 3, was also accused by Satan. Here in Revelation 12 we are told that the accusations were virtually nonstop. Probably, the accuser was demanding punishment for those believers on earth because they were still sinning. The death, resurrection and ascension of Jesus silenced him, which is what Paul says in Colossians 2:14-15.
Fourth, even although Jesus is triumphant over the powers of darkness, John wants his readers to realise that they have to fight on earth against those enemies and that sometimes his people will be martyred. It would have been possible for people to assume that since Jesus is on the throne, his people should not suffer. The reality is very different. Jesus from the throne calls on his people to witness for him, even unto death.
Fifth, the devil prowls the earth in an angry mood. His fury is not only directed at Christians. John refers to the earth and the sea as being the places where the devil is active, and earth and sea is another way of saying everywhere. The devil is here to destroy whatever he can. The chapter closes with him standing beside the sea waiting to do something or for something, which is detailed in the following chapter.