Peter & Paul have a common feast day. St. Leo, 5th century bishop of Rome, shows that this is because these apostles were united not only in faith, but shared a common martyrdom.
Precious in the eyes of the Lord is the death of his holy ones. No type of cruelty can tear down the religion established by the mystery of Christ’s cross. The Church is not diminished by persecutions, but rather increased. The field of the Lord is always being enriched with a more abundant harvest, while the seeds which are sown one by one yield a manifold return.
From this field those two famous shoots of the divine seed burst forth into a great progeny, witnessed by thousands of blessed martyrs. To emulate the apostles’ triumph, these martyrs have adorned our city far and wide with people clothed in purple and shining brilliantly, and they have crowned it with a diadem fashioned by the glory of many precious stones.
On the commemoration of all the saints it is right for us to rejoice in this heavenly band, fashioned by God as models of patience and a support for our faith; but we must glory and exult even more in the eminence of these two forebears, whom the grace of God raised to so high a summit among all the members of the Church, and established like two eyes that bring light to the body whose head is Christ.
As to their merits and virtues, which no words can describe, we should not think of any difference or distinction between them; their calling was the same, their labors were similar, theirs was a common death.
Our experience has shown, as our predecessors have proved, that we may believe and hope that in all the labors of the present life, by the mercy of God, we shall always be helped by the prayers of our special patrons. Just as we are humbled by our own sins, so we shall be raised up by the merits of these apostles.
This excerpt from a sermon of St. Leo the Great (Sermo 82, in natali apostolorum Petri et Pauli 1, 6-7: PL 54, 426-428) is used in the Roman Office of Readings for November 18, the memorial of the Dedication of the Basilicas of St. Paul Outside the Walls and St. Peter’s in Vatican City.