Letter CXLV belongs to a critical moment in the later Leo corpus. Emperor Marcian — the ruler who had convened the Council of Chalcedon in 451 and whose firm hand had suppressed the Eutychian opposition throughout his reign — died on January 26 or 27, 457. His death was the signal the Eutychian party had been waiting for. During Holy Week of that same year, the orthodox bishop Proterius of Alexandria was dragged from the baptistery, murdered by a mob, and his body desecrated. The monophysite Timothy Aelurus was installed in his place. The new emperor, Leo I, had no established Chalcedonian commitments. Pope Leo’s letter of July 11, 457 is his opening move in an engagement with that emperor at a moment of acute crisis.

The primacy dimension of this letter operates through the manner of Leo’s petition rather than through formal rulings or explicit jurisdictional language. When Leo writes that it would be glorious for the emperor to grant “to the universal Church at my supplication” what he has already seen fit to deny the heretics on his own account, the construction is precise: Leo is the one through whom the universal Church’s cause is presented to secular authority. This is not diplomatic modesty. Leo holds ordinary and immediate jurisdiction over the whole Church — not as a delegate of its constituent churches, but as the holder of Peter’s office from which governance flows to the entire body. There is no parallel track by which the emperor might receive the universal Church’s petition through some other channel; Leo’s supplication is the universal Church’s supplication, because the office makes it so.

The direction Leo gives the emperor at the close of Chapter II is equally significant beneath its diplomatic surface. Leo does not simply ask the emperor to appoint an orthodox bishop and leave the particulars to imperial discretion. He specifies the process — “through Catholic priests” — and the criteria: integrity of conduct and profession of the faith. This is the immediate jurisdiction of the Roman pontiff in operation over a church thousands of miles from Rome. Leo governs Alexandria’s episcopal succession by defining the terms of qualification; the emperor provides the enforcement. The Roman bishop’s authority reaches directly into the Alexandrian church — not through the Alexandrian church’s own structures, not through any intermediary patriarch, but immediately, with the emperor as the instrument of execution. This is what ordinary and immediate jurisdiction looks like in a fifth-century political context: it does not require Leo’s physical presence or a formal legation; it operates through the letter itself.

The Chalcedonian argument of Chapter I belongs to the same structure. The definitions of Chalcedon are inviolable not because councils are inherently self-sealing, but because this council was assembled through the Holy Spirit and its rule was brought forth by divine inspiration. The reader who follows this corpus carefully will recognize what underlies that claim: Leo confirmed Chalcedon’s acts as the expression of the apostolic faith he holds in Peter’s succession. What the Roman pontiff has confirmed on the basis of that succession is not subject to revision by any subsequent assembly. The council’s definitions are beyond reconsideration because the authority that made them definitive — the Apostolic See’s confirmation — is itself beyond challenge.

Letter CXLV is not the most theologically dense letter in the corpus, but its historical moment gives it exceptional weight. It is Leo’s opening communication to an untested emperor at the moment when the Chalcedonian settlement was most vulnerable since 449. The measured but structurally clear claim to represent the universal Church, combined with the precise direction on Alexandria’s episcopal succession, shows the ordinary and immediate jurisdiction of the Roman pontiff operating in the most natural register — exactly as one would expect of an authority that is constitutive of the Church’s structure rather than exceptional within it.