As Christians, we belong to the one, holy, catholic, and apostolic Church, simply called: the Catholic Church. While it is helpful shorthand, too often “Catholic” is treated as a proper title and not an adjective. “Catholic”, primarily, does not refer to a particular policy or membership. Instead, it is a description of the Church: “universally prevalent” (Oxford English Dictionary). This is applied to the Church as a whole and to any true Church within the Church Catholic. For the local parish church or the individual Christian, what does it mean to be “catholic”?
For that is truly and in the strictest sense Catholic, which, as the name itself and the reason of the thing declare, comprehends all universally. This rule we shall observe if we follow universality, antiquity, consent. We shall follow universality if we confess that one faith to be true, which the whole Church throughout the world confesses; antiquity, if we in no wise depart from those interpretations which it is manifest were notoriously held by our holy ancestors and fathers; consent, in like manner, if in antiquity itself we adhere to the consentient definitions and determinations of all, or at the least of almost all priests and doctors.Commonitory 1:6, St. Vincent de Lerins
This Vincentian Canon gets to true catholicity and can serve as a way of reflection. For example, do my morals only apply to some people? Is everyone called to this or that standard of holiness? Are my morals universal not just to space but also to time: does the Church of today let certain things “slide” that the Church of yesterday would have rebuked?
What about my doctrine? If catholicity is determined by what is believed always, everywhere, and by all, then false doctrine is marked by its novelty: it begins at a particular time, in a particular place, and with a particular person. Hence: Arianism, Americanism, and Modernism.
Catholicity serves as an effective measure and guardrail: it ties us to our roots, which go down to the Scriptures. We all are bound to follow the Scriptures, and the Scriptures reveal Jesus Christ’s promise:
When the Spirit of truth comes, he will guide you into all the truth, for he will not speak on his own authority, but whatever he hears he will speak, and he will declare to you the things that are to come.John 16:13
The Church, being the servant of the Scriptures, listens to the Scriptures in context: not just the context of the text itself but also the context of the history of the Holy Ghost guiding her.
It is this which makes the church catholic. And something cannot be declared or made “catholic” by a bishop or theologian. By the time he would try to make a novelty “catholic”, he is already too late.