For two reasons. First, the word means universal. Secondly, it really began to expand into the world after Paul was imprisoned in Rome and was able to tell the Gospel to the Roman Guard. YBIC
The "catholic" church was started in 325 ad and again in 525 ad. In 872 ad Bishop Thomas, the catholic bishop to Rome, decided that the present emperor was busy fighting a war and would not notice that he started his own religion. He declared himself pope and created the "roman catholic" religion. The "one holy and apostolic catholic church" is today called Greek orthodox.
It began as an insult actually and Catholics just took up the insult and kept it.
All Christians prior to the Reformation were Catholic. Even the Orthodox are Catholic just as all Catholics are Orthodox. This is because the words orthodox and catholic are adjectives not nouns. It isn't until Protestantism created denominations that they became nouns. In fact, without Protestantism they make no sense as nouns.
However, the church has been called catholic since the first century. While the earliest use of the term in writing we have that has survived comes from the year 103, it comes from Irenaeus, who was the third Patriarch of Antioch where Christianity was initially based from. He was trained by both John and Peter. He was ordained by Peter and succeeds as Peter's successor at Antioch. He not only knew the apostles, but he gives us an enormous amount of information about what the bible means to the original authors.
Scholars in petrology (study of the first Christians) believe the word was probably coined around AD 70 or AD 80. There are two reasons, first when it is used later it is used in such a way that it doesn't need explaining implying it was widely understood by the time we have surviving documents. Second, Paul has a section (I cannot remember which one off the top of my head) which basically means what the words Kata holy mean (now written as catholic). It is likely that passage from Paul, as well as certain ones in various Johannine writings that triggered the adjective.
Catholic has two meanings. Either it means "according to the whole," or it can also mean "all embracing." Unfortunately this often gets translated as Universal. While an "all embracing," body would by definition be universal, to be embraced carries a different co notation than being part of a universal entity.
As an all embracing body it accepts extraordinary amounts of diversity. There is more diversity within Catholicism than across all of Protestantism. There are forms of ancient Christianity present in Catholicism that are absent entirely in Protestantism. It attempts to embrace all. The second meaning is "according to the whole." Which means the Church holds as true only those teachings present in the Church held since the apostles across all time and place.
Certain words cannot be qualified or they have no meaning. Universal or according to the whole or all embracing would be one of them. The idea of someone being "Roman" Catholic implied that they were not Catholic but rather a limited form of Catholic which is of course impossible. How can something be limited and universal?
So it is called Roman as a Protestant insult and because by implication Protestants are not Christian. If Catholics are Catholic then Protestants are heretics because the world Catholic in essence means non-denominational. The idea of denominations are explicitly condemned in Revelations (see the Nicolatians) and in various letters of Paul where he talks about people following certain named people. The word denomination means to name. Lutherans follow Luther, Calvinists follow Calvin, Catholics embrace all. It was essential to self image to deny the Catholic Church any historical claim it was trying to embrace all for that would imply Protestants were not.
Unfortunately, the historical reality is that Protestant groups are exclusive. There are now 46,000 Protestant denominations, each one claiming to correctly understand the truth. The others are just wrong or very wrong. You have snake handlers, Quakers, Pentecostals, fundamentalists, Methodists, Presbyterians, Lutherans and so forth. Many of them are farther from each other than they are from Catholicism. The principal in Protestantism is that when you don't agree, you divide into a new group. How can a group embrace all when division is its operating principal?
It was necessary for the self image of Protestantism for Protestants to deny that Catholics were Christian. Just as the myth that Constantine had anything to do with the founding of the Catholic Church was important to some. Brad Ehrman, an evangelical theologian, writes a wonderful essay on Constantine and the falsehood of that idea in the popular press book he wrote on the Da Vinci code. He is an excellent scholar, he recently translated the early Christian writers and he wrote a wonderful book on scripture called "Misquoting Jesus."
Much of what people believe about Catholicism is completely false, but the Reformation was a civil war where 1/3rd of Germans died. It was necessary to attack, even with lies, Catholics because once princes started killing princes they could not permit any dissent.
The Catholic Church has consistently referred to itself as the ?Catholic Church? at least since 107 AD, when the term appears in the writings of St. Ignatius of Antioch
The term "Roman" Catholic is rather recent.
The new Anglican Church in England started using the term ?Roman? in the 1500s as one of many ways of demeaning and demonizing Catholics.
Catholics accepted this late coming adjective without too much protest. Today ?Catholic? and ?Roman Catholic? are interchangeable terms. Both terms are even used in the Catechism of the Catholic Church.
To add a little more confusion, some apply the term ?Roman Catholic Church? only to the Latin Rite Catholic Church, excluding the Eastern Rite Catholic Churches that are in full communion with the Pope, and are part of the same Church, under the Pope.
The term ?Roman? neither increases nor decreases the faith, hope and love of the Catholic Church.
Why is the Roman Catholic Church called 'Roman'?
