It was Pope Leo XIII himself who gave the order that it be recited in every church in the world at the end of Mass.

The  fascinating story of this prayer:

One hundred twenty-one years ago Pope Leo XIII (1810-1903), the pontiff of Rerum Novarum, the Church's first social encyclical, died. But Leo XIII was not only the “social” pontiff that most books of historiography rightly give us. He was, in fact, also the author of a particular prayer to St. Michael the Archangel, a veritable exorcism. And behind this prayer lies a fascinating story, all to be discovered, all to be remembered.

It was October 13, 1884 when Pope Leo XIII, while celebrating Mass in his private chapel, distinctly heard two voices. One, sweet; the other, harsh and harsh. The first voice was that of Jesus; the other of Satan. The dialogue between them was very animated: the evil one would ask Jesus for more time and power to be able to destroy the Church. The time required to carry out his plan was 75 years-100 years: Jesus would agree to the request, specifying, however, that the gates of Hell would certainly not have the last word.

That same mystical experience of Leo XIII was further enriched by a real vision. The pontiff himself described in this way what I had seen: “I saw the Earth enveloped in darkness and an abyss; I saw legions of demons coming out and spreading throughout the world to destroy the works of the Church and to attack the Church itself, which I saw reduced to the point of exhaustion. Then St. Michael appeared and drove the evil spirits back into the abyss. Then I saw St. Michael the Archangel intervene, not at that time, but much later, when people had multiplied their fervent prayers to the Archangel.” The pontiff was dumbfounded at such an apocalyptic scenario. As soon as he was aroused, Pope Leo XIII quickly reentered his office and wrote, on the spur of the moment, a prayer to St. Michael the Archangel.

The prayer, in its extended form, was later included in the Church's official exorcism book and as such, in cases of possession, was recited only by an authorized priest. In this version the prayer is very long, unlike that in the short form, which is better known. We quote only part of it: “[St. Michael], come to the aid of men created by God in his image and likeness and redeemed at great cost from the tyranny of the devil. Fight the battles of the Lord today with the whole army of the blessed angels, as you have already fought against the prince of pride Lucifer and his apostate angels; and the latter failed to triumph, and now there is no place for them in Heaven. But this great dragon has fallen, this ancient serpent who is called the devil and Satan, who sets traps for everyone.” Then, reference was made to what the pontiff had seen, “Now behold, this ancient enemy, this old murderer, stands again with renewed anger.” And, further on, it closed with a plea for help to St. Michael the Archangel to combat the snares of the evil one.

Then there is, indeed, the shorter version: the one known today and recited by most of the faithful. It was Pope Leo XIII himself who gave the order that it be recited in all the churches of the world at the end of Mass, in the context of the so-called Leonine Prayers, a series of solemn prayers and invocations to God and the Virgin Mary, already in use since 1859. The prayer written and introduced into the Roman Ritual by Leo XIII, in the short form, read as follows : “In the name of the Father, the Son and the Holy Spirit,/ St. Michael the Archangel, defend us in battle, be our protection against the wickedness 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.”

With the Second Vatican Council and the abolition of the Preci Leonine, a first drastic limitation of the prayer written by Leo XIII will take place. This reform would be followed by other limitations over the years until the very important prayer almost fell into oblivion. It would be taken care of in 1994 by St. John Paul II to remind the faithful of the precious words of Pope Leo XIII: “May prayer strengthen us for that spiritual battle of which the Letter to the Ephesians speaks: ‘Draw strength from the Lord and the vigor of his power’ (Eph. 6:10). It is to this same battle that the Book of Revelation refers, recalling before our eyes the image of St. Michael the Archangel (cf. Rev. 12:7). Pope Leo XIII certainly had this scene well in mind when, at the end of the last century, he introduced throughout the Church a special prayer to St. Michael” (Regina Coeli, April 24, 1994).

In 2018, Pope Francis reminded God's people of the famous prayer in a Holy See press release, dated Sept. 29, 2018: “The Holy Father has decided to invite all the faithful, from all over the world, to pray the Holy Rosary every day, during the entire Marian month of October; and thus to unite in communion and penance, as God's people, in asking the Holy Mother of God and St. Michael the Archangel to protect the Church from the devil, who always aims to divide us from God and from each other. [...] The Holy Father also asked that the recitation of the Holy Rosary during the month of October be concluded with the prayer written by Leo XIII.”