Una Petri Sedes
The preeminent role of the See of Rome as custodian and guarantor of the one orthodox faith — that of Nicaea — was formally acknowledged by the imperial authority in an edict on 27 February 380, during Damasus' episcopate (366-384). This was the famous edict promulgated by Theodosius at Thessalonica. In it the emperor — mandating the profession of the Catholic Faith for all the peoples (cunctos populos) under his authority — indicating as the absolute reference point the religion which "the divinely inspired Apostle Peter passed on to the Romans and which the Pontiff Damasus and Peter, Bishop of Alexandria, follow"; they who believe this doctrine — the Imperial Constitution adds — should take the name Christiani catholici; all the others are heretics and as such they ought to fear not only the divine chastisement but also the imperial punishment (Codex Tbeodosianus, 16.1.2).
It was an epochal event which expedited the action of Damasus — who was energetically supported by Ambrose, Bishop of Milan — to consolidate, jurisdictionally in particular, the primacy of the Church of Rome. Then an unforeseen and preoccupying obstacle to this process occurred, which was certainly not favourable to the Roman cause: the Third Canon from the Council of Constantinople in 381, which deliberated on a primacy of honour — after Rome — for the Constantinopolitan Church and its titulary, for the new capital founded along the Bosphorus was now the "New Rome" (Verumtamen Constantinopolitanus episcopus habeat honoris primatum praeter Romanum episcopum, propterea quod urbs ipsa sit iunior Roma).
Following these deliberations, — between the spring and summer of 382 — Damasus convoked in Rome another conciliar assembly, in which the major metropolitans of the West participated — including Ambrose — but none of the bishops of the East, who clearly opposed and objected, as formulated in the Epistola Constantinopolitani concilii ad papam Damasum (Theodoret, Ecclesiastical History 5 : 9).
The fundamental result of this council session was the decisive and irrevocable response to the third canon of the Council of Constantinople, clearly antagonistic to the Roman See. For the first time in a council — as conveyed by the third chapter of the so-called Decretum Gelasianum — the theological foundation for the primacy of the Bishop of Rome and St Peter's direct successor was formally justified with the interpretation in a juridical and disciplinary sense of the passage from Mt 16:17 (tu es Petrus et super hanc petram aedificabo ecclesiam meam), and no longer only — as Constantinople claimed — with a primacy of rank and honour.
In a similar vein, the Roman Council in 382 expressed itself in terms which admitted no debate: "Yet the Roman Church is not set ahead of other churches by any synodal decrees but rather by the Gospel's words of Our Lord and Saviour did it take hold of the primacy: thou art, he says, Peter" (sancta tamen Romana ecclesia nullis synodicis constitutis ceteris ecclesiis praelata est sed evangelica voce Domini et salvatoris nostri primatum obtinuit: tu es, inquit, Petrus). Damasus, as Charles Pietri noted, "tenaciously opposed the authority of apostolic tradition to the growth of a political capital (Constantinople)".
In Rome, during the years prior to the triennium of 380-382, Damasus had already begun to put together a network of consent and diffusion of the conceptualization of Roman primacy, making use of the instrument of written communication, which was most suitable to him: systematic memorial inscriptions. Indeed, with a shrewd plan of action, on the Vatican Hill itself — close to the Basilica dedicated to the Apostle — he had an inscription composed in hexameter in the baptistery; the final verses (the only ones extant) work out the unbreakable convergence of two unities as a theological synthesis: baptismal initiation and the Petrine Primacy of the See of Rome. The expressive forms and implied meaning of this composition — said Damasus with his characteristic loftiness of personality — are upheld and recommended almost exclusively to him by Peter himself. This much we can gather from what remains of the epigram, so solemnly
stated: "but by the preeminent Peter, to whom the gate of heaven was consigned, Damasus — bishop of Christ — composed [this]; one is Peter's see; there is one true cleansing [by baptism]" (sed praestante Petro cui tradita ianua caeli est, antistes Christi conposuit Damasus una Petri sedes, unum verum[que] lavacrum in the Epigrammata damasiana, rec. et adn. A. Ferrua, Vatican City 1942, n. 4).
The concept that connected the oneness of Baptism and of the primacy of the Church of Rome received a further development of form in the epitaph — in verses — dedicated appropriately to Damasus'
successor, Siricius (384-399), as the eloquent figura of the sedes apostolica. The deceased Pontiff s written memorial opens with the presentation of his ecclesiastical cursus and contemporaneously with the explicit affirmation of the legitimacy of apostolic succession, which through his immediate predecessors, Liberius and Damasus, reaches its origin: Peter. "He, as lector and then soon after as deacon, followed Liberius after Damasus, [and was] distinguished during all the years he lived; he deserved to sit upon the sacred fount as High Priest": Liberium lector mox et levita secutus post Damasum clarus totos quos vixit annos, fonte sacro magnus meruit sedere sacerdos (Inscriptiones Christianae Urbis Romae, IX, 24832). The author is reworking Damasus' concept lavacrum/sedes Petri, gleaning from it the image of fons sacer which here, without any doubt, assumes the role of a special titulus of the Roman See. However, this incorporation, stemming from the theological and symbolic elaboration during the pontificates of Damasus and of Siricius, did not subsequently find a place in the pontifical epigraphy of Rome; nevertheless, it found an echo of support during the second half of the fourth century in pictorial production.
On the basis of the symbolic assimilation of Moses-Peter in the cemetery figures, in sarcophagi and — not by chance — in objects which had a widespread circulation, such as gilded glass objects, a new iconography was formalized: inspired by the ancient legend, it represented Peter in the act of making water flow miraculously from the rock, toward which two soldiers — the Apostle's jailors at the Mamertine Prison, Processus and Martinianus — advanced in order to drink and thus to be converted.