Pope Leo reportedly received this prayer during a vision he received just after celebrating Mass on October 13, 1884. In it the devil had requested the time and power to destroy the Church. The time given by Jesus to him was approximately 75 to 100 years and he was given free reign over those who chose to follow him. We do not know when that time began or if it has ended.
O glorious Prince of the heavenly host, Saint Michael the Archangel, defend us in the battle and in the fearful warfare that we are waging against the principalities and powers, against the rulers of this world of darkness, against the evil spirits. Come you to the assistance of men, whom Almighty God created immortal, making them in His own image and likeness and redeeming them at a great price from the tyranny of Satan. Fight this day the battle of the Lord with your legions of holy Angels, even as of old you did fight against Lucifer, the leader of the proud spirits and all his rebel angels, who were powerless to stand against you, neither was their place found any more in heaven. And that apostate angel, transformed into an angel of darkness who still creeps about the earth to encompass our ruin, was cast headlong into the abyss together with his followers. But behold, that first enemy of mankind, and a murderer from the beginning, has regained his confidence. Changing himself into an angel of light, he goes about with the whole multitude of the wicked spirits to invade the earth and blot out the Name of God and of His Christ, to plunder, to slay and to consign to eternal damnation the souls that have been destined for a crown of everlasting life. This wicked serpent, like an unclean torrent, pours into men of depraved minds and corrupt hearts the poison of his malice, the spirit of lying, impiety and blasphemy, and the deadly breath of impurity and every form of vice and iniquity. These crafty enemies of mankind have filled to overflowing with gall and wormwood the Church, which is the Bride of the Lamb without spot; they have laid profane hands upon her most sacred treasures. Make haste, therefore, O invincible Prince, to help the people of God against the inroads of the lost spirits and grant us the victory. Amen.
We are always to test the spirits as St. John calls us to in his letter. 1 John 4:1-6.
In my experience it is by spending time with Jesus in the Most Blessed Sacrament that we begin to become sensitive to the spiritual warfare going on around us as well as within us. He will teach us how to be good soldiers in this spiritual war. He will give us the weapons and defences we need.
Remember, the greatest gift we have been given, one of the ones Satan really hates, is the fact that we can fall, and then through an act of contrition and the Sacrament of Reconciliation be back in the good graces of God! Praise be Jesus Christ for this wonderful gift!