St. Augustine on the sects wishing to be called Catholic

And so, lastly, does the very name 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.

St. Augustine (354–430): Against the Epistle of Manichaeus called Fundamental, chapter 4: Proofs of the Catholic Faith

Philip Schaff on Sects and the Church

Sects then owe it to themselves, as soon as they have fulfilled their historical vocation, to fall back again to the general Church communion from which they have seceded, as in no other way can their spiritual acquisitions be either completed or secured, and they must themselves otherwise stiffen into monumental petrifacations, never to be revisited with the warm life pulse of the one universal Church.

Principle of Protestantism by Philip Schaff