On the morning of October 13, 1884, Pope Leo XIII had just finished celebrating Mass in his private chapel in the Apostolic Palace. As he descended the altar steps, he suddenly stopped, his face turned grey, and he stood for some minutes in a state of profound abstraction — staring fixedly in front of him, his attendants alarmed.
When he came to himself he went immediately to his study and wrote what would become the great Leonine Prayer to St. Michael: 'Sancte Michael Archangele, defende nos in proelio...' ('Holy Michael Archangel, defend us in battle, be our protection against the wickedness and snares of the devil...'). He ordered that the prayer be recited at the end of every Low Mass throughout the Catholic world (a practice that continued until the 1965 liturgical reforms suppressed the prayers at the foot of the altar).
Multiple later accounts — from Cardinal Nasalli Rocca (later, in 1946), from sister Carmelite of Lisieux Sister Marie of the Eucharist (Thérèse's cousin), and from Father Domenico Pechenino who was present — record Leo XIII's own subsequent explanation: in those minutes after Mass he had heard, simultaneously, two voices conversing — one harsh and mocking, asking permission to attempt the destruction of the Church; the other gentle and full of authority, granting it for a period (variously reported as 'a century' or '75 to 100 years'). The prayer to St. Michael was Leo's immediate response. Pope John Paul II revived the practice of reciting the prayer at the conclusion of the Angelus.