“For the solidity of that faith which was praised in the chief of the Apostles is…”

For the solidity of that faith which was praised in the chief of the Apostles is perpetual: and as that remains which Peter believed in Christ, so that remains which Christ instituted in Peter.

Leo I · The Letters and Sermons · NPNF2 XII, Sermon III

Preached on an anniversary of his own consecration. It is the Roman claim stated as a syllogism rather than a boast — the see is what it is because the faith Peter confessed does not decay — and it is the sentence every account of how the fifth-century papacy argued its case comes back to.