Letter XX is among the shortest in the Leo corpus — a brief acknowledgment of a report received, a word of praise, and a promise of action — but it may be the most consequential short letter Leo ever wrote. It is the first document in what will become the defining episode of his pontificate: the Eutychian controversy, which will generate the Tome of Leo (Letter XXVIII), the Council of Chalcedon (451), and the definitive Christological settlement of the ancient Church. Understanding who Eutyches is, and what he had written to prompt this reply, is essential for the reader who wishes to follow the arc of the Leo corpus from this point forward.

Eutyches was the archimandrite of a large monastery near Constantinople — a man of considerable influence at the imperial court, closely connected to the powerful eunuch Chrysaphius, and long known as an energetic opponent of Nestorianism. His report to Leo, which this letter acknowledges, warned that Nestorian ideas were reviving in some quarters of the East. Leo’s response is entirely sympathetic: he praises Eutyches’s zeal and promises to act once he knows who the perpetrators are. There is no hint of suspicion. At this moment Leo and Eutyches are on the same side — or appear to be.

The irony that the reader of the full corpus will immediately feel is this: within months, Eutyches himself will be condemned at a local synod in Constantinople for teaching the opposite of Nestorianism — not that Christ was two persons but that after the Incarnation He had only one nature, the divine absorbing the human. It will be Eutyches who appeals to Leo for rehabilitation. And it will be Leo — who here promises to root out the Nestorian poison — who instead confirms the condemnation of Eutyches and issues the Tome, the most authoritative Christological document in the Western Church, defining against both Nestorianism and Eutychianism the Catholic faith in two natures united in one person. Letter XX is the moment just before the controversy breaks open. Leo is responding to a trusted informant’s alarm; he does not yet know that the informant is himself the problem he will be called to address.

For the question of papal primacy, Letter XX matters in a different way. Leo is writing to a presbyter-monk in Constantinople — a major city in the Eastern Empire, the seat of the emperor, the home of the patriarch whose see had been elevated by the Council of Constantinople in 381. It is worth noting for the reader that Rome never accepted the disciplinary canons of that council — only its contribution to the Creed was received as authoritative in the West — so that the elevation of Constantinople’s patriarch to a primacy of honor “second after Rome” rested on a conciliar canon Rome had not ratified. After Chalcedon (451), the same issue would resurface in an even sharper form: Canon 28 of that council extended Constantinople’s privileges further, assigning it equal honor with Rome on the basis of its status as the imperial city. Leo rejected Canon 28 explicitly and in writing, and the canon was nullified in the West — though it remained the basis of Constantinople’s claims in the East, and is to this day a point of division between the Roman Catholic and Eastern Orthodox understandings of ecclesial structure. All of that lies ahead at the time of Letter XX. For now, the significant fact is simply that Eutyches writes to Leo rather than to his own patriarch as the authority to whom the revival of heresy must be reported — a datum about how the Roman see’s universal solicitude was understood even by those closest to the rival see that would eventually contest it most directly.