In the second half of the fourth century, Byzantius, the Roman senator, and Saint Pammachius, his son, fashioned their house on the Cælian Hill into a Christian basilica.

In the fifth century the presbyteri tituli Byzantii (priests of the church of Byzantius) are mentioned in an inscription and among the signatures of the Roman Council of 499.

[1] In the ancient apartments on the ground-floor of the house of Byzantius, which were still retained under the basilica, the tomb of two Roman martyrs, John and Paul, was the object of veneration as early as the fifth century.

[1] The Sacramentarium Leonianum already indicates in the preface to the feast of the saints, that they rested within the city walls, while, in one of the early itineraries to the tombs of the Roman martyrs, their grave is assigned to the church on the Cælian (de Rossi, Roma sotterranea, I, 138, 175).

[2] According to their Acts, which are legendary and may not be historically reliable, after successful military careers, Constantine the Great entrusted John and Paul with the protection of his daughter, Constantia.

Constantine bequeathed significant sums to the faithful of a John and Paul, who built a house on the Caelian Hill and retired to private life, doing works of charity.

[5] The rooms on the ground-floor of the above-mentioned house of Pammachius were rediscovered in the 19th century under the Basilica of Santi Giovanni e Paolo in Rome.

[4] The Lüneberg manuscript (c. 1440–1450) mentions the day of John and Paul in an early German account of the Pied Piper of Hamelin: In 1284 on the day of John and Paul on 26th June 130 children born in Hamelin were led away by a piper [clothed] in many colours to [their] Calvary near the Koppen, [and] lost'.