This has been presented before on this blog, but it bears repeating to understand. A Pope had a vision of satan unleashed on the church. He composed a prayer for the aid of St. Michael the Archangel.
There was a longer version of the prayer and a shorter version. The shorter version, which people say today, is this.
Saint Michael the Archangel, defend us in battle, be our protection against the malice and snares of the devil. May God rebuke him we humbly pray; and do thou, O Prince of the Heavenly host, by the power of God, thrust into hell Satan and all evil spirits who wander through the world for the ruin of souls. Amen.