From Dom Guéranger's The Liturgical Year.
The commemoration of the illustrious Pope and Martyr, Stephen I, adds a perfume of antiquity to the holiness of this day dedicated to the honour of a comparatively modem saint. Stephen’s special glory in the Church is to have been the guardian of the dignity of holy baptism. Baptism once given can never be repeated; for the character of child of God, which it imprints upon the Christian, is everlasting; and this unspeakable dignity of the first sacrament in no wise depends upon the disposition or state of the minister conferring it. According to the teaching of St. Austin, whether Peter, or Paul, or Judas, baptize, it is He upon whom the divine Dove descended in the Jordan, it is He alone and always that baptizes by them in the Holy Ghost. Such is the adorable munificence of our Lord, with regard to this indispensable means of salvation, that the very pagan who belongs not to the Church and the schismatic or heretic separated from her can administer it with full validity, on the one condition of fulfilling the exterior rite in its essence, and of wishing to do thereby what the Church does.
In the time of Stephen I this truth was not so universally known as now. Great bishops, whose learning and holiness had justly won them the admiration of their age, wished to make the converts from various sects pass again through the laver of salvation. But the assistance promised to Peter was not wanting to his successor; and by maintaining the traditional discipline, Rome, through Stephen, saved the faith of the churches. Let us testify our gratitude to the holy pontiff for his fidelity in guarding the sacred deposit, which is the treasure of all men; and let us beg him to preserve no less effectually in us also the nobility and the rights of our holy baptism.
Prayer
Deus, qui nos beati Stephani martyris tui atque pontificis annua solemnitate lætificas: concede propitius; ut cujus natalitia colimus, de ejusdem etiam protectione gaudeamus. Per Dominum.
O God, who givest us joy by the annual solemnity of blessed Stephen, Thy martyr and bishop, mercifully grant that we may rejoice in the protection of him whose festival we celebrate. Through our Lord, etc.