That's a legitimate question. I've answered it to some degree, but there are no easy answers to that question. And over the 1500 years of history which get cited repeatedly, there are many points to discuss.
There are a few easy-to-define moments, such as the conversion of Constantine, and the moment, as Eamon Duffy put it, "Bishops all over the Roman world would now be expected to take on the role of judges, governors, great servants of state." ("Saints and Sinners: A History of the Popes" New Haven: Yale Nota Bene Press, 1987, 2001, pg 37). During that era, the leadership of the church was incorporated into the imperial bureaucracy and effectively became a functionary of the imperial government. Shortly thereafter Constantine decamped and "the Church" was left as the only game in town. It was easy for them to write their own rules. And they did. Rules that were different from what the earlier church had practiced.
In that way, the flip side of that question is also legitimate. Contra Newman, who suggested that “the Christianity of the second, fourth, seventh, twelfth, sixteenth, and intermediate centuries is in its substance the very religion which Christ and His Apostles taught in the first”, it is fair to say that the Roman church never was fully on the trail. Or rather, it was creating its own trail: one that did not have all of its roots in older Christianity.
I do think it is fair to say that, if early church history were to be studied in detail, no one group would be entirely happy with what was found. Yet that doesn't preclude any Protestant group from claiming the earlier church as part of its own heritage.
One individual who had a thorough knowledge of the Reformation was Jaroslav Pelikan. It was Pelikan who edited and catalogued Luther's Works in English. [And yes, I know that Pelikan converted to Eastern Orthodoxy at the end of his life. That will be another discussion.] Pelikan said: "Recent research on the Reformation entitles us to sharpen it and say that the Reformation began because the reformers were too catholic in the midst of a church that had forgotten its catholicity," said Jaroslav Pelikan, "The Riddle of Roman Catholicism," New York: Abingdon Press, 1959, pg 46. But he goes even further than this:
That generalization applies particularly to Luther and to some of the Anglican reformers somewhat less to Calvin, still less to Zwingli, least of all to the Anabaptists. But even Zwingli, who occupies the left wing among the classical reformers, retained a surprising amount of catholic substance in his thought, while the breadth and depth of Calvin's depth to the heritage of the catholic centuries is only now beginning to emerge.I think it's important to note at this point that "opposition to Roman domination" is as important as having "taught evangelical doctrine." There is a two-fold need: (a) kick the supports out from underneath the domineering Roman behemoth, and (b) understand evangelical -- Biblical -- doctrine as it appeared throughout the centuries. Continuing with Pelikan:
It is important to make clear what we mean by "the heritage of the catholic centuries." The reformers were catholic because they were spokesmen for an evangelical tradition in medieval catholicism, what Luther called "the succession of the faithful." The fountainhead of that tradition was Augustine (d. 430). His complex and far-reaching system of thought incorporated the catholic ideal of identity plus universality, and by its emphasis upon sin and grace it became the ancestor of Reformation theology. … All the reformers relied heavily upon Augustine. They pitted his evangelical theology against the authority of later church fathers and scholastics, and they used him to prove that they were not introducing novelties into the church, but defending the true faith of the church.
Not only Augustine could serve to substantiate the claim of the reformers to be truly catholic. Throughout the centuries they found substantiation. Although they spoke of the "fall of the church" in the post-apostolic era, they seized upon individuals and groups in every epoch of Christian history who had opposed Roman domination or who had taught evangelical doctrine. (Pelikan, 46-47).
Bernard of Clairvaux (d. 1153) was practically canonized by the reformers for his opposition to Rome. They also managed to find more obscure figures in medieval history. To prepare books like the Magdeburg Centuries they combed the libraries and came up with a remarkable catalogue of protesting catholics and evangelical catholics, all to lend support to the insistence that the Protestant position was, in the best sense, a catholic position.Probably nowhere was this "claim to catholicity" more apparent than when the Reformers cited "the Gospel." This was not a new "Gospel," it is one that Christ and the Apostles preached. Based on work that I've cited from T.F. Torrance and others, the admixture of the necessity of "works" into the gospel was not biblical, but a later accretion. The truest conformity to the Apostolic preaching was held by the Reformers. "Substantiation for this understanding of the gospel came principally from the Scriptures, but whenever they could, the reformers also quoted the fathers of the catholic church. There was more to quote than their Roman opponents found comfortable" (Pelikan 48-49).
Additional support for this insistence comes from the attitude of the reformers toward the creeds and dogmas of the ancient catholic church. The reformers retained and cherished the doctrine of the Trinity and the doctrine of the two natures in Christ which had developed in the first five centuries of the church….
If we keep in mind how variegated medieval catholicism was, the legitimacy of the reformers' claim to catholicity becomes clear. With men like Augustine and Bernard on their side, the reformers could well protest against the usurpation of the name "catholic church" by their opponents" (47-48)
In the end, the Council of Trent ended up (in true Roman fashion) condemning the true heritage, and canonizing its own path. In its decrees, Trent "selected and elevated to official status the notion of justification by faith plus works, which was only one of the doctrines of justification [found] in the medieval theologians and ancient fathers. When the reformers attacked this notion in the name of the doctrine of justification by faith alone -- a doctrine also attested to by some medieval theologians and ancient fathers-- Rome reacted by canonizing one trend [the wrong one] in preference to all the others. What had previously been permitted (justification by faith and works), now became required. What had been previously been permitted also (justification by faith alone), now became forbidden. In condemning the Protestant Reformation, the Council of Trent condemned [the better part of] its own catholic tradition" (Pelikan 51-52, Pelikan's comments (in parentheses), my own comments in [square brackets]).
Lord willing we'll explore all of this in further detail.