Your question implies that doctrine and theology in Christianity has been generally static in the history of Christianity before the Protestant Reformation. Truth, is, the religion of Christianity we now call today took many lines long before the Protestant Reformation happened.
The Roman Catholic Church, today is a witness to the The East-West Schism in 1054 that separated the Roman Catholic Church and the Eastern Orthodox Church. Though it sounds like they separated suddenly at that date, historians contend that the roots of the separation between the East and the West Churches were sown long before. Generally, they had some disagreements on theology such as the filioque clause dispute on whether the Holy Spirit proceeds from the Father and the Son, or from the Father only and had roots before the East-West Schism.
The Monophysite controversy also is a significant event. People before debated whether Jesus Christ had two natures, divine or human, or had only a single nature which was either divine, or a mix of divine or human natures. The Council of Chalcedon in 451 AD settled these, declaring the doctrine of hypostatic union, and the Roman Catholic Church and the Eastern Orthodox Church we know today can be considered as a part of Chalcedonian Christianity from this period that believed in the Council of Chalcedon. Those who digressed from this are the Oriental Orthodox Churches we now know today such as Coptics, they are "Miaphysites", not really Monophysite but close since they believe Jesus Christ is of a single nature only.
So, people back then disagreed whether Christ had one or two natures. They debated whether the Holy Spirit came from the Father or the Father and the Son. I'm not sure whether this makes sense for you, but this gives a picture for you that Christianity is not static before the Reformation and had many convergences in theology and doctrine long before it. The Catholic position in Rome was not the only position, such as on the filioque and the nature of Christ, many people disagreed and did not believed.
This may obviously answer your question, as you can see a lot of people there who are not Catholics, such as Eastern Orthodox Christians, Coptics, who belong to Miaphysites, Oriental Orthodox people, giving you an idea that Christianity was diverse and varied long before the Reformation.
As for the term "Catholic", it came from the Greek "katholou". and can refer to a lot of things, such as "universal" or "all-embracing", and many also use it to refer to the Apostolic Christian tradition. Many use it such as the Roman Catholic, Eastern Orthodox, Anglican, so MaesterMagoo's answer is valid as well. The earliest recorded use of "Catholic" was in the Letter to the Smyrnaeans (8:2) by Ignatius of Antioch in 107 AD, so we can see that the use of the term is very old as well and entrenched in history.
Source:
"Great Schism" in Oxford Dictionary of the Christian Church, Oxford University Press, 2005.
The Rise of the Monophysite Movement, W.H.C Frend, 1972
No. Christianity is all the religions that use teachings of one Jesus Christ as the basis, catholic is one denomination of it, specifically one that is in full communion with Pope in Rome (that is, agrees he is their boss). There are several rites in the catholic church, most common being Roman rite, hence Roman Catholic. Rites are just local customs though, the underlying doctrine and faith is the same.
As for protestants, Christianity split apart long, long before them. Eastern orthodox church is the most well known, but there were many, many others, like monophysites or assyrians.