The appearance of the Archangel was a direct response to the pope's unceasing prayers for his people.
Near the end of the 6th century, a severe plague ravaged the city of Rome, even claiming the life of Pope Pelagius II. It was a difficult time for the people of the city, and when Pope Gregory I was elected to lead the Church, he immediately set out to call upon the mercy of God.
St. Gregory the Great (as he would later be known) organized a massive procession around the city, inviting everyone to pray to God that the plague would be lifted. The Golden Legend narrates how the procession was led by an ancient image of the Virgin Mary, reportedly cleansing the air of disease.
The plague was still ravaging Rome, and Gregory ordered the procession to continue to make the circuit of the city, the marchers chanting the litanies. An image of Blessed Mary ever Virgin was carried in the procession … And lo and behold! The poisonous uncleanness of the air yielded to the image as if fleeing from it and being unable to withstand its presence: the passage of the picture brought about a wonderful serenity and purity in the air. We are also told that the voices of angels were heard around the image, singing Regina coeli laetare, alleluia,
Quia quem meruisti portare, alleluia,
Resurrexit sicut dixit, alleluia!
to which Gregory promptly added: Ora pro nobis, Deum rogamus, alleluia!
As the procession continued around the city, St. Gregory reached the Mausoleum of Emperor Hadrian and saw a sight that brought peace to his soul.
Then the pope saw an angel of the Lord standing atop the castle of Crescentius, wiping a bloody sword and sheathing it. Gregory understood that that put an end to the plague, as, indeed, happened. Thereafter the castle was called the Castle of the Holy Angel.
Eventually a statue of St. Michael the Archangel, sheathing his sword, was put on the top of Castel Sant’Angelo, which remains a constant reminder of the mercy of God and how he responded to the prayers and supplications of the people.
When God sees a united and sincere faith, he responds generously to answer their prayers.