Petrine Cathedra is the Guarantor of Catholicity

He who "communicates" with Rome, "communicates" with the world: he who does not "communicate" with Rome, does not "communicate" with the world. Rome is, so to speak, the proof-bearing indicator of whether one is in communion with the universal Church. --Plinthos translation

Quien "comulga" con Roma, "comulga" con el mundo; quien no "comulga" con Roma, no "comulga" con el mundo. Roma es, por así decir, el indicio comprobable de si se está en comunión con la Iglesia universal.

Joseph Ratzinger, Obras Completas I (The 1951 Thesis on Augustine), 129-130.