As a Christian ecclesiastical term, Catholic - from the Greek adjective καθολικός, meaning "general" or "universal" ^[1] - is described in the Oxford English Dictionary as follows:

~Church, (originally) whole body of Christians; ~, belonging to or in accord with (a) this, (b) the church before separation into Greek or Eastern and Latin or Western, (c) the Latin Church after that separation, (d) the part of the Latin Church that remained under the Roman obedience after the Reformation, (e) any church (as the Anglican) claiming continuity with (b)."^[2]

Leaving aside the historical meanings indicated under (b) and (c) above, the Oxford English Dictionary thus associates present-day Catholicism with:

(a) "the whole body of Christians". The actual extension of Catholicism in this sense varies with the different understandings of what it means to be a Christian.

(d) "the part of the Latin Church that remained under the Roman obedience after the Reformation", i.e. the Catholic or Roman Catholic Church. This definition of Catholicism should be expanded to cover the Eastern particular Churches that are in full communion with the Bishop of Rome, and that the Church in question sees as no less part of Catholicism than the Latin particular Church.

The earliest surviving evidence of the use of the term "Catholic Church" is a letter that Ignatius of Antioch wrote in about 107 to Christians in Smyrna (Letter to the Smyrnaeans, .[1] Saint Ignatius used the term to designate the Christian Church possessing true traditions, excluding heretics, such as those who "confess not the Eucharist to be the flesh of our Saviour Jesus Christ, which suffered for our sins, and which the Father, of His goodness, raised up again" (Smyrnaeans, 7).[2] Exhorting Christians to remain closely united with their bishop, he wrote: "Wherever the bishop shall appear, there let the multitude [of the people] also be; even as, wherever Jesus Christ is, there is the Catholic Church" (Smyrnaeans .

Yet more explicit was the manner in which Cyril of Jerusalem (circa 315-386) used the term "Catholic Church" precisely to distinguish it from other groups that also claimed the title of "Church": "If ever thou art sojourning in cities, inquire not simply where the Lord's House is (for the other sects of the profane also attempt to call their own dens houses of the Lord), nor merely where the Church is, but where is the Catholic Church. For this is the peculiar name of this Holy Church, the mother of us all, which is the spouse of our Lord Jesus Christ, the Only-begotten Son of God."^[3]

Only slightly later, Saint Augustine of Hippo (354-430) wrote:

"In the Catholic Church, there are many other things which most justly keep me in her bosom. The consent of peoples and nations keeps me in the Church; so does her authority, inaugurated by miracles, nourished by hope, enlarged by love, established by age. The succession of priests keeps me, beginning from the very seat of the Apostle Peter, to whom the Lord, after His resurrection, gave it in charge to feed His sheep (Jn 21:15-19), down to the present episcopate.

"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.

"Such then in number and importance are the precious ties belonging to the Christian name which keep a believer in the Catholic Church, as it is right they should ... With you, where there is none of these things to attract or keep me... No one shall move me from the faith which binds my mind with ties so many and so strong to the Christian religion... For my part, I should not believe the gospel except as moved by the authority of the Catholic Church."

— St. Augustine (354–430): Against the Epistle of Manichaeus called Fundamental, chapter 4: Proofs of the Catholic Faith [3]

On 27 February 380, by an edict issued in Thessalonica and published in Constantinople, Emperor Theodosius declared Catholic Christianity the official religion of the Roman Empire, and defined the term "Catholic" in Roman Imperial law as follows:

Theodosian Code XVI.i.2:

It is our desire that all the various nations which are subject to our clemency and moderation, should continue the profession of that religion which was delivered to the Romans by the divine Apostle Peter, as it has been preserved by faithful tradition and which is now professed by the Pontiff Damasus and by Peter, Bishop of Alexandria, a man of apostolic holiness. According to the apostolic teaching and the doctrine of the Gospel, let us believe in the one Deity of the Father, Son and Holy Spirit, in equal majesty and in a holy Trinity. We authorize the followers of this law to assume the title Catholic Christians; but as for the others, since in our judgment they are foolish madmen, we decree that they shall be branded with the ignominious name of heretics, and shall not presume to give their conventicles the name of churches. They will suffer in the first place the chastisement of divine condemnation and the second the punishment of our authority, in accordance with the will of heaven shall decide to inflict.^[4]

A contemporary of Augustine, St. Vincent of Lerins, wrote in 434 under the pseudonym Peregrinus a work known as the Commonitoria ("Memoranda"). While insisting that, like the human body, Church doctrine develops while truly keeping its identity (sections 54-59, chapter XXIII), he stated: "[I]n the Catholic Church itself, all possible care must be taken, that we hold that faith which has been believed everywhere, always, by all. 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" (section 6, end of chapter II).