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From the time that any other adjective was applied to the noun “Church,” though, the term was derived not from the name of the divine Founder of this Church,but rather from one of its special characteristics: its catholicity. This body was the one, unique, saving body for everyone, everywhere, as distinguished from the partial or even counterfeit groups or sects that, already in New Testament times (1 John 2:19), were springing up in opposition to the true Church. They have not ceased to spring up in every age since.

Rather, the NT church itself was a small sect, considered counterfeit according to the historical magisterium. And which simply did not hold to distinctive Catholic beliefs according to the manifest light of the only wholly inspired substantive authoritative record of what the NT church believed.

And as Basil testifies, their was must division within.

Basil of Ceasarea, the ascetic 4th century Greek bishop of Caesarea:

Liberated from the error of pagan tradition through the benevolence and loving kindness of the good God, with the grace of our Lord Jesus Christ, and by the operation of the Holy Spirit, I was reared from the very beginning by Christian parents. From them I learned even in babyhood the Holy Scriptures which led me to a knowledge of the truth.

When I grew to manhood, I traveled about frequently and, in the natural course of things, I engaged in a great many worldly affairs. Here I observed that the most harmonious relations existed among those trained in the pursuit of each of the arts and sciences; while in the Church of God alone, for which Christ died and upon which He poured out in abundance the Holy Spirit, I noticed that many disagree violently with one another and also in their understanding of the Holy Scriptures.

Most alarming of all is the fact that I found the very leaders of the Church themselves at such variance with one another in thought and opinion, showing so much opposition to the commands of our Lord Jesus Christ, and so mercilessly rendering asunder the Church of God and cruelly confounding His flock that, in our day, with the rise of the Anomoeans, there is fulfilled in them as never before the prophecy, "˜Of your own selves shall men arise speaking perverse things, to draw away disciples after them.´

Witnessing such disorders as these and perplexed as to what the cause and source of such evil might be, I at first was in a state, as it were, of thick darkness and, as if on a balance, I veered now this way, now that"”attracted now to one man, now to another, under the influence of protracted association with these persons, and then thrust in the other direction, as I bethought myself of the validity of the Holy Scriptures.

After a long time spent in this state of indecision and while I was still busily searching for the cause I have mentioned, there came to my mind the Book of Judges which tells how each man did what was right in his own eyes and gives the reason for this in the words" "˜In those days there was no king in Israel.´ With these words in my mind, then, I applied also to the present circumstances that explanation which, incredible and frightening as it may be, is quite truly pertinent when it is understood; for never before has there arisen such discord and quarreling as now among the the members of the Church in consequence of their turning away from the one, great, and true God, only King of the universe... St. Basil : Ascetical Works Fathers of the Church, Volume 9

, And for some time heretics became the party in majority, and Roman unity was much thru popes who were more like "Christianized" Caesars than Peters.

Eamon Duffy (Former president of Magdalene College and member of Pontifical Historical Commission, and current Professor of the History of Christianity at the University of Cambridge) and provides more on the Roman church becoming more like the empire in which it was found as a result of state adoption of (an already deformed) Christianity:

The conversion of Constantine had propelled the Bishops of Rome into the heart of the Roman establishment...They [bishops of Rome] set about [creating a Christian Rome] by building churches, converting the modest tituli (community church centres) into something grander, and creating new and more public foundations, though to begin with nothing that rivaled the great basilicas at the Lateran and St. Peter’s...

These churches were a mark of the upbeat confidence of post-Constantinian Christianity in Rome. The popes were potentates, and began to behave like it. Damasus perfectly embodied this growing grandeur. An urbane career cleric like his predecessor Liberius, at home in the wealthy salons of the city, he was also a ruthless power-broker, and he did not he did not hesitate to mobilize both the city police and [a hired mob of gravediggers with pickaxes] to back up his rule…

Self-consciously, the popes began to model their actions and their style as Christian leaders on the procedures of the Roman state. — Eamon Duffy “Saints and Sinners”, p. • .

Upon Pope Liberius's death September 24 A.D. 366, violent disorders broke out over the choice of a successor. A group who had remained consistently loyal to Liberius immediately elected his deacon Ursinus in the Julian basilica and had him consecrated Bishop, but the rival faction of Felix's adherence elected Damasus, who did not hesitate to consolidate his claim by hiring a gang of thugs, storming the Julian Basilica in carrying out a three-day massacre of the Ursinians

On Sunday, October 1 his partisans seized the Lateran Basilica, and he was there consecrated. He then sought the help of the city prefect (the first occasion of a Pope in enlisting the civil power against his adversaries), and he promptly expelled Ursinus and his followers from Rome. Mob violence continued until October 26, when Damasus's men attacked the Liberian Basilica, where the Ursinians had sought refuge; the pagan historian Ammianus Marcellinus reports that they left 137 dead on the field. Damasus was now secure on his throne; but the bishops of Italy were shocked by the reports they received, and his moral authority was weakened for several years....

Damasus was indefatigable in promoting the Roman primacy, frequently referring to Rome as 'the apostolic see' and ruling that the test of a creed's orthodoxy was its endorsement by the Pope.... This [false claim to] succession gave him a unique [presumptuous claim to] judicial power to bind and loose, and the assurance of this infused all his rulings on church discipline. — Kelly, J. N. D. (1989). The Oxford Dictionary of Popes. USA: Oxford University Press. pp. 32 ,34;

More by God's grace.