When the Mormons used to come around I used to ask them the exact same question, surprise, surprise they could never answer it either.
That is easy as it was progressive,though it will take some space to briefly it.
We can see the deformation from the NT church in substance early on, progressively taking on things which were not seen in the NT and in contrast to it. Even if such was done with seemingly right motive, but as with the Jewish magisterium, progressively thinking of itself above that which is written. (1Cor. 4:6)
Paul Johnson, author of over 40 books and a conservative popular historian, writes,
The Church was now a great and numerous force in the empire, attracting men of wealth and high education, inevitably, then, there occurred a change of emphasis from purely practical development in response to need, to the deliberate thinking out of policy.
This expressed itself in two ways: the attempt to turn Christianity into a philosophical and political system, and the development of controlling devices to prevent this intellectualization of the faith from destroying it. The twin process began to operate in the early and middle decades of the third century, with Origen epitomizing the first element and Cyprian the second.
The effect of Origen’s work was to create a new science, biblical theology, whereby every sentence in the scriptures was systematically explored for hidden [much prone to metaphorical] meanings, different layers of meanings, allegory and so forth.....
Cyprian [c. 200 – September 14, 258] came from a wealthy family with a tradition of public service to the empire; within two years of his conversion he was made a bishop. He had to face the practical problems of persecution, survival and defence against attack. His solution was to gather together the developing threads of ecclesiastical order and authority and weave them into a tight system of absolute control...the confession of faith, even the Bible itself lost their meaning if used outside the Church.
With Cyprian, then, the freedom preached by Paul and based on the power of Christian truth was removed from the ordinary members of the Church, it was retained only by the bishops, through whom the Holy Spirit still worked, who were collectively delegated to represent the totality of Church members...With Bishop Cyprian, the analogy with secular government came to seem very close. But of course it lacked one element: the ‘emperor figure’ or supreme priest...
[Peter according to Cyprian was] the beneficiary of the famous ‘rock and keys’ text in Matthew. There is no evidence that Rome exploited this text to assert its primacy before about 250 - and then...Paul was eliminated from any connection with the Rome episcopate and the office was firmly attached to Peter alone...
...There was in consequence a loss of spirituality or, as Paul would have put it, of freedom... -(A History of Christianity, by Paul Johnson, pp. 51 -61,63. transcribed using OCR software) Then we have further deformation in periods as that of Damasus 1 (366-384) who began his reign by employing a gang of thugs in seeking to secure his chair, which carried out a three-day massacre of his rivals supporters. ...Ammianus Marcellinus reports that they left 137 dead on the field. (J. N. D. Kelly, “The Oxford Dictionary of Popes” (Hardcover), pp. 32 )
Just like the NT in Acts. Yet Rome made him a "saint."
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;
Moving along,
The sixth century found Rome sunk too low by war and pestilence for many churches to be built; but at this time took place the transformation of ancient buildings into Christian shrines. Instead of despising the relics of paganism, the Roman priesthood prudently gathered to themselves all that could be adopted from the old world. Gregorovius remarks that the Christian religion had grown up side by side with the empire, which this new power was ready to replace when the Emperor withdrew to the East.
The Bishop of Rome assumed the position of Ponlifex Maximus, priest and temporal ruler in one, and the workings of this so-called spiritual kingdom, with bishops as senators, and priests as leaders of the army, followed on much the same lines as the empire. The analogy was more complete when monasteries were founded and provinces were won and governed by the Church. - Welbore St. Clair Baddeley, Lina Duff Gordon, “Rome and its story” p. 176
Then you had those times it seems so many FR RCs seem to long for.
in the 1180s, the Church began to panic at the spread of heresy, and thereafter it took the lead from the State, though it maintained the legal fiction that convicted and unrepentant heretics were merely 'deprived of the protection of the Church', which was (as they termed it) 'relaxed', the civil power then being free to burn them without committing mortal sin. Relaxation was accompanied by a formal plea for mercy; in fact this was meaningless, and the individual civil officer (sheriffs and so forth) had no choice but to burn, since otherwise he was denounced as a 'defender of heretics', and plunged into the perils of the system himself. — Paul Johnson, History of Christianity, © 1976 Athenium, p. 253
Canons of the Ecumenical Fourth Lateran Council, 1215:Secular authorities, whatever office they may hold, shall be admonished and induced and if necessary compelled by ecclesiastical censure, that as they wish to be esteemed and numbered among the faithful, so for the defense of the faith they ought publicly to take an oath that they will strive in good faith and to the best of their ability to exterminate in the territories subject to their jurisdiction all heretics pointed out by the Church; so that whenever anyone shall have assumed authority, whether spiritual or temporal, let him be bound to confirm this decree by oath.
But if a temporal ruler, after having been requested and admonished by the Church, should neglect to cleanse his territory of this heretical foulness, let him be excommunicated by the metropolitan and the other bishops of the province. If he refuses to make satisfaction within a year, let the matter be made known to the supreme pontiff, that he may declare the ruler’s vassals absolved from their allegiance and may offer the territory to be ruled lay Catholics, who on the extermination of the heretics may possess it without hindrance and preserve it in the purity of faith; the right, however, of the chief ruler is to be respected as long as he offers no obstacle in this matter and permits freedom of action. - http://www.fordham.edu/halsall/basis/lateran4.asp
More .
And besides previous messy times, we have the schism of the 14th and 15th centuries, of which Cardinal Ratzinger observed,
"For nearly half a century, the Church was split into two or three obediences that excommunicated one another, so that every Catholic lived under excommunication by one pope or another, and, in the last analysis, no one could say with certainty which of the contenders had right on his side. The Church no longer offered certainty of salvation; she had become questionable in her whole objective form--the true Church, the true pledge of salvation, had to be sought outside the institution. It is against this background of a profoundly shaken ecclesial consciousness that we are to understand that Luther, in the conflict between his search for salvation and the tradition of the Church, ultimately came to experience the Church, not as the guarantor, but as the adversary of salvation. — Joseph Cardinal Ratzinger, head of the Sacred Congregation of the Doctrine of the Faith for the Church of Rome, “Principles of Catholic Theology,” trans. by Sister Mary Frances McCarthy, S.N.D. (San Francisco: Ignatius, 1989) p.196 ).
..."one pope (Gregory XII) had voluntarily abdicated; another (John XXIII) had been suspended and then deposed, but had submitted in canonical form; the third claimant (Benedict XIII) was cut off from the body of the Church, "a pope without a Church, a shepherd without a flock" (Hergenröther-Kirsch). It had come about that, whichever of the three claimants of the papacy was the legitimate successor of Peter, there reigned throughout the Church a universal uncertainty and an intolerable confusion, so that saints and scholars and upright souls were to be found in all three obediences. On the principle that a doubtful pope is no pope, the Apostolic See appeared really vacant, and under the circumstances could not possibly be otherwise filled than by the action of a general council." - http://www.newadvent.org/cathen/04288a.htm
We also had the hindrance of Rome to free access of and literacy of the Scriptures, as a latter development.
Finally we come to the time of the Reformation, which prior history and what follows provides context for, which while far from perfect for sure, was necessary and set multitudes of captives free to know the Lord and greatly expand the kingdom of God thru the centuries, including thru America, and thus was a blessing for Catholics as well.
And not that the church has ceased before that, as it still professed the truths by which faith comes, (Rm. 10:17) but which souls had to see thru the trapping of institutionalized religion. Yet souls were yet save in it, as is the case today, with the one true church being the body of Christ, as was the case in Scripture, not just the church at Ephesus, and none looked to an infallible pope in Rome as its supreme exalted head.
But Rome increasingly ceased to be the viable visible manifestation of that body, as is the case today, and while not totally apostate, it overall has become as the gates of Hell for her multitudes, myself having been one of them. Thanks be to God.
Cardinal Bellarmine: "Some years before the rise of the Lutheran and Calvinistic heresy, according to the testimony of those who were then alive, there was almost an entire abandonment of equity in ecclesiastical judgments; in morals, no discipline; in sacred literature, no erudition; in divine things, no reverence; religion was almost extinct. — Concio XXVIII. Opp. Vi. 296- Colon 1617, in “A History of the Articles of Religion,” by Charles Hardwick, Cp. 1, p. 10,
• Erasmus, in his new edition of the “Enchiridion,” “What man of real piety does not perceive with sighs that this is far the most corrupt of all ages? When did iniquity abound with more licentiousness? When was charity so cold?” — “The Evolution of the English Bible: A Historical Sketch of the Successive,” p. 132 by Henry William Hamilton-Hoare
“Probably as many as half the men in orders had ‘wives’ and families. Behind all the New Learning and the theological debates, clerical celibacy was, in its own way, the biggest single issue at the Reformation. It was a great social problem and, other factors being equal, it tended to tip the balance in favour of reform. As a rule, the only hope for a child of a priest was to go into the Church himself, thus unwillingly or with no great enthusiasm, taking vows which he might subsequently regret: the evil tended to perpetuate itself.” (Johnson, History of Christianity, pgs 269-270)