Where was the Church during the Middle Ages? Was it Rome? Was it hidden within Rome?
Or has our doctrine of the Church caused us to miss the fact that there were thousands, sometimes even tens of thousands of people outside the Roman communion during the Medieval Period? They were standing for a Bible-based Christianity and rejecting the theology of Christendom...the Holy Society, the Church-State project that brought us Roman Catholicism, the Holy Roman Empire, the Inquisition and the Crusades.
These groups of Christians before the Reformation are referred to as proto-Protestants, as if they were prototypes of what was to come after 1517.
The history is confusing and not always clear cut. In addition to Protoprotestants like the Waldensians, Lollards, and Hussites, you have heretical groups like the Cathars or Albigenses who also rejected Rome. They for the most part were not teaching a form of Biblical Christianity.
You also have groups like the Lollards who initially wanted the English crown to reform the Church. As great as Wycliffe was, he was flawed in his embrace of this hope. After the Peasant's Revolt of 1381, and the so-called Oldcastle Rebellion of 1415, the Lollards went underground and seemed to abandon their Constantinian aims. The 15th century was one of death and persecution for them and as they became more grounded in Scripture, they came to reject part and parcel the entire Roman system.
The Hussites, (named after the martyr Jan Huss) in Bohemia, Moravia, Silesia, and Hungary were split into several factions. The Utraquists only wanted a minor reform and were largely content to stay in communion with Rome. The Taborites were Bible-based, but succumbing to Nationalism took up the sword and warred against their Roman Catholic and German oppressors who held the reins of the Holy Roman Empire. The Taborites shook Christendom as their one eyed and eventually blind general Zizka led them to victory after victory. After many years, the sad tale ends with the fall of the Taborites. Many Protestants have mistakenly glorified their deeds.
But not all. Even in their day there were groups of proto-Protestants that rejected the Taborite Sword. Petr Chelcicky is probably the best known and some of his followers along with ex-Taborites eventually formed the group known as the Unitas Fratrum or Jednota Bratrska....the Unity of the Brethren. They did not join with the Reformation but maintained a distinct identity and though they are not the same groups by any means, we know their descendants today as the Moravian Church.
There were many other groups as well. The Middle Ages are little known to the modern reader, often cast in very simplified and monolithic terms. They were every bit as complex as our world today with a myriad of social and political arrangements and dynamics in tension. Everyone was not a Roman Catholic...that's too simple. Even the Feudalism we read of in text books is an oversimplified explanation of the Medieval order.
There are two streams of Protestant historical interpretation that I'm rejecting....
One, that Rome was the True Church during the Middle Ages but ceased to be at the time of the Reformation when Luther clarified the gospel and the Council of Trent rejected it. The succession and claim of valdity to be the True Church then passed to the Protestant Communions.
This is historically fallacious and rooted in theological error. Much time is given to these matters in the posts at this site. Rome had ceased to be a Church centuries before.
Two, that the proto-Protestant groups were exactly the same in doctrine to the later Protestant groups. Lutheran and Reformed theology were just continuations of what the proto- groups stood for. This is also a mistake.
I'm arguing many of the proto-Protestant groups were diverse and many embraced a surprisingly developed theology constructed from Scripture and rejected much of the Scholasticism of the period. Scholasticism was repudiated in the Humanistic climate of the Reformation, but re-embraced (in modified form) by the later Protestant theologians. As a result, much of Protestant theology has become rooted in System rather than Scripture.
In addition, Protestantism embraced the Constantinianism of the Middle Ages and went about the same task, creating a Protestant Christendom.
These two issues....theological method, and anti-Constantinianism were lost when the vast majority of proto-Protestants joined with the Reformation.
At the time of the Reformation there were some proto-Protestants who joined with a new movement known as the Anabaptists, who later became the Mennonites and Amish we know today. They shed their previous theology for a more consistent theological system, though one rooted more in reason and experience than Scripture. And it was these Anabaptists who almost alone continued the anti-Constantinian heritage of proto-Protestantism.
So while not embracing the majority of their theology, they shouldn't be ignored and we can learn much from their interactions with the Protestant communions who were every bit as hostile and persecuting as the Roman Inquisition had been.
Only later with the Mayflower Pilgrims and men like Roger Williams do we begin to see a conscientious Protestant rejection of Constantinianism. But it didn't last long. In the New World, the temptations of power and the desire to build a New Jerusalem led the American Church to re-embrace the same old Constantinianism albeit in modified form.