Originally, the word “catholic” was used to note that one was talking about the Church spread throughout the world as opposed to a local Church (Church of Ephesus, Church of Corinth, Church of Antioch, etc.–what we think of now as a diocese, which is really a regional boundary where a local Church–a bishop, his clergy, and his flock celebrating the Eucharist–resides). As schisms and heresies arose even from the earliest time, such separated churches and communities were considered no longer part of this one Church spread throughout the world–they were no longer part of the catholic Church. Since the Church is one, there can only be one catholic Church, as we profess in the Creed.
Despite centuries of schisms and heresies and even the many sins of its own members, this one catholic Church remains in all its integrity as founded by Christ.
And so, lastly, does the name itself of Catholic, which, not without reason, amid so many heresies, the Church has thus retained; so that, though all heretics wish to be called Catholics, yet when a stranger asks where the Catholic Church meets, no heretic will venture to point to his own chapel or house.