It's a familiar story: a call to reform is nailed to the door of an important church, specifically objecting to the excesses of the medieval Catholic hierarchy, and it galvanizes a movement that opposes the Church's purported monopoly on salvation.
That specific sequence of events is most famous from Martin Luther's 95 Theses, but it's also precisely the story of an English document penned over a century earlier. Calls for a decentralized Christianity that embraces popular knowledge of theology rather than the governance of a single hierarchy that claims authority on all matters of religion were abundant even before the time when we now recognize the start of the Protestant Reformation.
To some degree the Reformation truly began in the twelfth century, with the rise of the Waldensians in southern France. Teaching that the whole of God's will could be found in the Bible, the early Waldensians asserted that it was improper for any member of the clergy to gain worldly power, that pilgrimages were without spiritual effect, that the Bible does not require abstinence from meat on certain days, and that holy water is without effect. The Waldensians did not, originally, reject the necessity of priests, though they did reject the high-church style and even the need for consecrated church buildings.
The Waldensians were persecuted by the Catholic Church - including by means of a papal bull calling for a crusade to exterminate them in 1487; they even came to identify the Church as the Antichrist. The sect survived this pressure, eventually joining the followers of Calvin and Zwingli in the Reformed movement. There are, in fact, Waldensian churches within the Reformed tradition to this day, and modern Waldensians embrace the fundamentals of Calvin's theology.
In the later Middle Ages, confidence in the Church was shaken further by the Western Schism, and so it is not surprising that other reform movements arose. Most notable among them were the English Lollards (a name probably taken from a Dutch word for "mumblers"), who echoed the Waldensians on many points, but were also substantially more radical. In the Twelve Conclusions of the Lollards, a brief and anonymous document nailed to the door of Westminster Abbey in 1395, they extend certain Waldensian tenets. Not only did they believe the Bible was the sole source of the word of God, they held that each person was their own authority on reading it, and that there was thus a moral imperative to have the Bible available in the vernacular. (This resulted in the translation and creation of beautiful manuscript Bibles in northern dialects of Middle English, known as "Wycliffe Bibles" after John Wycliffe, the probable founder of the Lollard sect.) The Lollards opposed temporal offices being held by the clergy, but they also preached the equality of all social classes; Lollard preacher John Ball is famous for asserting that God created men equally, with no serfs or lords, saying "When Adam delved and Eve span, who then was the gentleman?" The Lollards further rejected the high-church institutions criticized by the Waldensians, opposing pilgrimage, asserting that if holy water were truly holy it would be a perfect cure-all (which it observably isn't), and even claiming that exorcisms and the consecration of religious artifacts are a form of black magic. They also held that an artfully-decorated church was specifically a celebration of the Old Testament approach to venerating God which Christ had made unnecessary.
The Lollards also openly denied the doctrine of transubstantiation (something the earlier Waldensians were also accused of, though it's not clear this was accurate), holding that Christ is eternally in Heaven and so rejecting the notion that a human priest could draw him out of it into the bread and wine of communion. (They did, however, continue to celebrate mass, complete with a communion in commemoration of Christ.) Following Wycliffe, the Lollards also regarded the practice of selling indulgences as a form of simony, a charge which would be echoed by more famous protestant movements in later centuries. They also rejected the sacrament of confession, holding that it serves primarily to give the priests more power over their parishioners; they further held that this power makes Christians afraid to give an honest confession, thereby tempting them into sin.
The Lollards were also nearly absolute pacifists, holding that "manslaughter by battle or law of righteousness for temporal cause or spiritual with out special revelation is express contrary to the New Testament, the which is a law of grace and full of mercy." They even condemned the institution of knighthood, on the grounds that it gives men worldly glory for their success at killing and that this is an affront to God. In attacking both serfdom and knighthood as diabolical institutions, the Lollards set themselves, on purely theological grounds, against the entire edifice of medieval social organization. And the nominal pacifism of at least certain leaders of the movement did not stop Lollardy from being the guiding theology in a violent commoners' uprising which seized the Tower of London in the year 1381. (In fact, John Ball was among the leaders of the Great Rising, and preached a sermon on social equality to the assembled masses immediately before they marched on London.) To the rebels, social inequality was an affront to God, but how to end it was a question of temporal politics. Advocating the "Law of Winchester," they sought to free all serfs and have each village govern itself, upending the entire structure of feudal society. The tradition of Piers Plowman literature, which comes mostly out of the Lollard movement, criticizes both the excesses of the Catholic church and the inequality of feudal society; as discussed in my last post, this poetic tradition further shaped the rhetoric of the Great Rising and was used to help stir the masses to revolt. While the Lollard movement would then be stamped out in the early 15th century, their writings lived on and influenced much of the doctrine, if not the politics, of later Protestantisms in ways which continue to this day.
(It should be noted that the Great Rising was a populist movement which had appeal among both Lollards and Catholics. While Ball's preaching fell on receptive ears among those involved in the rebellion, he does seem to have kept it on the political side, rather than preaching about the finer points of the sacraments. Likely that's specifically because the revolt was about politics rather than theology, and the rebels' theological views were probably quite varied. But it's also quite likely that the political side of Lollardy and in particular the radicalism of John Ball exerted a strong influence on the other leaders of the Great Rising.)
Now, it's not clear how universal the details of particular doctrines were within the Lollard movement. Since Wycliffe preached, and Lollardy as a whole held, that each man was charged to read the Bible as an individual, there was of necessity some variation in understanding of theology from one Lollard to another. But the specific variations are ill-documented and mostly unavailable to us. And the variation certainly did not stem from ignorance; unlike the anti-epistemology of certain later sects such as the Abecedarians, the Lollards did not deny the relevance of expertise in knowledge of the Bible. Rather, they held that it was necessary for each believer to become an expert for their own sake. So it is that Lollardy came to include specific denunciations of the Church on points of theology, such as the nature of communion, while expecting each person within the movement to understand and espouse in detail the arguments for the Lollard view. In fact, this spreading of knowledge of detailed theology which Lollardy required is why views on transubstantiation came to be seen as a litmus test for orthodoxy among English Catholics. For teaching in favor of social justice, for instance, Geoffrey Chaucer was sometimes accused of being a secret Lollard; he rebuts this assertion in the Canterbury Tales by introducing the poor Parson, a village priest who in most regards fits every stereotype attached to Lollardy. And when his turn comes to tell a story, the Parson shows his radical adherence to his own reading of Scripture by quoting a denunciation of fiction by the apostle Paul. The audience, then, is primed to see him presented as a heretic - but the tale he tells is a sermon in which he spells out the traditional Catholic case for the real presence of Christ during the sacrament of communion. In this, Chaucer proves that both author and character hold with orthodoxy. But he also demonstrates that a fully orthodox Catholic can and should affirm the Lollard concern for the welfare of the downtrodden in society, a message at once subtler and more subversive than any amount of radical preaching.