From Dom Guéranger's The Liturgical Year.
On this day whereon Satan, for the first time, sees his infernal crew fall back in face of the sacred Ark, two warriors of the army of the elect take their rank in our Queen's cortège. Deputed by Peter himself, during this his glad octave, to wait upon Mary, they have earned this honour by reason of their faith, which taught them to recognize in Nero's condemned criminal the chief of God's people.
The prince of the apostles was awaiting his martyrdom in the dungeon of the Mamertine prison, when, led by divine mercy, there came to him two Roman soldiers, whose names have become inseparable from his own in the Church's memory. One was called Processus, the other Martinianus. They were struck by the dignity of the old man, confided for some hours to their ward, who would not see daylight again until he was led out to execution. Peter spoke to them of life eternal, and of the Son of God who so loved men as to give the last drop of his Blood for their ransom. Processus and Martinianus received with docile heart this unexpected instruction; they accepted it with simple faith, and craved the grace of regeneration. But water was wanting in the dungeon, and Peter was forced to make use of the power to command nature, bestowed by our Lord upon the apostles when he sent them into the world. At the word of the old man a fountain sprang up from the ground, and the two soldiers were baptized in the miraculous water. Christian piety still venerates this fountain, which never either brims over or dries up. Processus and Martinianus soon paid with their life for the honour conferred upon them of being thus initiated into the Christian faith by the prince of the apostles, and they are numbered among God's martyrs.[27]
Their cultus is as ancient as that of Peter himself. In the age of peace, a basilica was raised over their tomb. St Gregory pronounced there, on the solemn anniversary of their combat, his thirty-second homily on the Gospel. The great Pontiff therein renders testimony to the miracles which were operated on that holy spot, and he celebrates, in particular, the power which those two saints have of protecting their devout clients on the day of the Lord's justice.[28] Later on, St Paschal I enriched the basilica of the prince of the apostles with their bodies. They now occupy the place of honour in the left arm of the Latin cross formed by the immense edifice, and they give their name to the whole of this side of the transept, wherein the Vatican Council held its immortal sessions; it was fitting that this august assembly should carry on its labours under the patronage of these two valiant warriors, who were not only St Peter's guards, but his conquest in the days of his own glorious confession. Let us not forget these illustrious protectors of holy Church. The feast of the Visitation, of more recent institution, has not lessened theirs; though their glory is now, so to say, lost in that of our Lady, their power can but have gained in strength by this approximation to the gentle Queen of earth and heaven.