Letter V is an encyclical — one of a number of identical letters Hilarius dispatched after his accession to announce his elevation to the Apostolic See. The one preserved here is addressed to Leontius of Arles; others of identical form would have gone to the leading bishops of every province. Such encyclicals are the formal means by which a new pope announced his accession to the universal Church and requested the prayers of his fellow bishops. The form is conventional; what is worth the reader’s attention is the theological framing Hilarius chose to place at the opening.

The very first sentence, before any mention of Hilarius’s own elevation, establishes the reverence owed to Peter and to his See as the settled premise of the Catholic tradition. The phrasing is careful: the reverence is not proposed or defended — it is “known to all who hold the incorrupt guardianship of paternal traditions.” This is not a new claim being advanced but the ecclesiological ground on which everything else in the letter (and in Hilarius’s pontificate) rests. The reader should notice that Peter and his See are treated as a single object of reverence — beato Petro apostolo et sedi ipsius. The same construction appears throughout Leo’s correspondence: Peter is present in his See, and the See’s authority is Peter’s continuing authority.

The specific office Hilarius names as his own is apostolicae sedis regimen — “the governance of the Apostolic See.” The word regimen is the language of rule, of active governance. This is not an honorific description; it is the self-designation of the office Hilarius now holds. And the scope of that office is named in the letter’s closing purpose clause: the prayers of the Gallic bishops are to profit the universal Church. The reader may compare Leo’s parallel language in Letter V (to the Illyrian metropolitans), where the Roman bishop’s sollicitudo is described as owed to all the Churches. Hilarius is using the same theological vocabulary for the same claim: the office he has received is universal in scope, and its governance is not provincial or regional but reaches to the whole Church. That this claim appears in the very first letter of Hilarius’s pontificate — in its opening sentence, before any other business — shows that it is the foundational premise of everything that follows.