Letter IX is Hilarius’s first application of the Gallic conciliar structure he had established less than a year earlier in Letter VIII. The case is Mamertus of Vienne’s forcible occupation of the city of Die and consecration of a bishop there — a jurisdictional violation of the first order, because Die was not within Vienne’s province and had been formally assigned to a different jurisdiction by the Apostolic See. The letter’s primacy content is unusually administrative, and that administrative register is itself worth the reader’s attention.

The most striking passage is the naming of the Apostolic See as the authority that had assigned the jurisdictional boundaries — ex apostolicæ sedis deputavit auctoritas — paired with the citation of Rome’s archives as the documentary record: sicut in scriniis nostris tenemus. The claim is twofold. First, the territorial organization of the Gallic churches was not established by local custom, conciliar agreement, or the accumulation of historical usage; it was assigned by Apostolic See authority. Second, Rome holds the official record of those assignments. When a bishop crosses a canonical boundary, the answer to the question “whose jurisdiction was it?” is found in the Roman archives. This is primacy in its most administrative form: Rome as the registrar of ecclesiastical geography, maintaining the official documentation of the canonical map of the Latin West.

The procedural handling is equally revealing. Mamertus’s offense is described as manifold, and Hilarius explicitly says he could have pronounced judgment directly — the facts as reported would have supported it. But he defers: moderation requires it, the canonical order requires it, and Leontius had spoken in terms that suggested a more measured handling. So Hilarius refers the case to the annual synod that Letter VIII had established, with Leontius presiding. The procedure is precise: the council will investigate, demand an accounting from Mamertus, report collectively to Rome, and receive Rome’s final order suppressing the illicit consecration. The conciliar body has real investigative authority; the final juridical act — ordinemus, “We may order” — belongs to the Apostolic See. This is the same pattern visible throughout Leo’s Gallic correspondence and throughout Hilarius’s own rulings: provincial inquiry, Roman definition.

The sequence of the correspondence is itself significant. Letter VIII established the Gallic conciliar structure in December 462. Letter IX, dated October 463, puts that structure to work for the first time. The interval is short — ten months — and the case Hilarius assigns to it is a major jurisdictional dispute involving an established metropolitan bishop. The conciliar order Hilarius created is therefore not merely nominal; it is the operative mechanism by which Gallic ecclesiastical matters are to be handled, under papal oversight, with Roman archives supplying the documentary basis for jurisdictional determinations.