Start with what he was actually complaining about, because it was a specific financial arrangement and not an abstraction.
An indulgence remitted the temporal punishment due for sins already confessed and forgiven, which in practice meant time in purgatory. The theology behind it was elaborate and had been developed over centuries. The practice by 1517 was that you could obtain one by giving money to an authorised cause.
The campaign running in Germany that year was authorised to fund the rebuilding of St Peter's in Rome. It had a second purpose that was not advertised. A young nobleman named Albrecht already held two bishoprics and wanted the archbishopric of Mainz as well, which required a papal dispensation because it broke the rules on holding multiple sees and because he was under age. The dispensation was granted for an enormous fee, which Albrecht borrowed from the Fugger banking house of Augsburg. Half the proceeds of the indulgence campaign in his territories went to Rome, and half went to repay the bank.
The friar preaching it, Johann Tetzel, was an effective salesman, and the jingle attributed to him, that as soon as the coin in the coffer rings the soul from purgatory springs, may or may not be his words but was certainly the impression given.
So a professor of biblical theology objected. On the thirty first of October 1517 he sent a set of ninety five propositions to Archbishop Albrecht, and possibly also posted them for academic debate, which is what such lists were for.
They are in Latin, numbered, and written as debating points, not as a manifesto. Anyone expecting a call to revolution is in for a surprise.
Most of them are technical: what the pope can and cannot remit, whether purgatory is within his jurisdiction, what repentance means. Several assume the pope would disapprove of the abuses if he knew. The first states that the whole life of believers should be repentance, which is a claim about the inner disposition rather than about transactions.
The sharpest are the ones with a public edge. Why does the pope, who is richer than Croesus, not build St Peter's with his own money rather than that of poor believers. Why does he not empty purgatory out of holy love, if he can. If indulgences can remit punishment, then the man who gives to a beggar does better than the man who buys one.
That is the argument that travelled. Somebody translated the theses into German and printed them, without the author's involvement, and within weeks they were circulating across the empire. This is chapter twenty three in action: the technology existed, the trade routes existed, and a short vernacular text about money and salvation was exactly what the market wanted.
What turned a dispute about indulgences into a schism was the sequence of demands that he retract, each of which forced him to state a more fundamental position in order to justify the previous one.
Challenged in debate at Leipzig in 1519 to distance himself from Jan Hus, who had been burned as a heretic a century earlier, he examined the case and said that some of Hus's positions had been Christian and that councils could err. That is heresy by definition, and everybody in the room understood it.
By 1520 he was publishing the underlying position in three short books. Salvation comes through faith, given by God, not earned by works or purchased. Scripture is the authority, above popes and councils, and must therefore be available to be read. And every baptised Christian is a priest, so the clergy are a function, not a caste with special access.
Take those three together and the institution described in chapters nine, fourteen and seventeen has no necessary role. That, and not the indulgence, is the reason it could not be settled.
He was excommunicated, burned the bull publicly, and was summoned in 1521 before the emperor and the imperial diet at Worms, where he refused to recant. The words most often quoted, that here he stood and could do no other, appear in the printed version and not in the earliest reports, and are probably an addition. What he did say was that his conscience was captive to the word of God and that going against conscience is neither safe nor honest, which is the more consequential sentence anyway.
Reformers had been condemned before. Wycliffe in England and Hus in Bohemia had made overlapping arguments in the previous century, and Hus was burned at a church council despite a safe conduct. The difference in 1517 is not the theology.
Printing, first. The output figures for these years are extraordinary: hundreds of editions of his works within a few years, in cheap pamphlets, in German, illustrated with woodcuts for the illiterate. He became the most published author in Europe by an enormous margin, and the church's replies were slower, longer and in Latin.
Politics, second. Germany was not a state but hundreds of principalities under an emperor with limited power. His own prince, the elector of Saxony, protected him, partly out of conviction and partly because a famous professor was an asset and because Rome's fiscal demands on Germany were resented by everyone. Other princes did the arithmetic on church lands, which were extensive, and which a reformation would transfer to them.
Distraction, third. The emperor Charles the Fifth, who ruled Spain, the Netherlands, Austria and much of Italy, and who wanted the movement crushed, spent the following decades fighting France and holding off Ottoman advances that reached Vienna in 1529. He could rarely bring force to bear in Germany, and when he finally won a war there in 1547 it was too late.
Once the principle is that scripture is the authority and every believer may read it, the obvious question is who decides what it means, and the answer turned out to be everybody.
In Zurich a preacher reached similar conclusions independently and went further on the sacrament, and the two movements failed to agree at a conference in 1529 on precisely that point, which fixed a division inside Protestantism that has never closed.
In Geneva a French lawyer built the most systematic and most exportable version, with a rigorous theology of predestination and a disciplined church structure that could operate without a friendly prince. That is what made it portable, and it is why the Calvinist form spread into France, the Netherlands, Scotland, Hungary and England while the Lutheran form largely stayed where princes adopted it.
And at the radical end, groups who concluded that infant baptism was unscriptural and that the church should be a voluntary body of believers separate from the state. They were persecuted by Catholics and Protestants alike. In 1534 a group seized the city of Münster, declared the millennium, instituted community of goods and polygamy, and were besieged and destroyed with great brutality. The episode was used for centuries as the argument against religious radicalism of any kind.
The moment that clarified where Luther himself stood came in 1525, when peasants across southern Germany rose with a list of demands, several of them quoting his own writings on Christian freedom. He answered with a pamphlet urging the princes to smite, slay and stab the rebels like mad dogs. Perhaps seventy to a hundred thousand were killed. Whatever the reformation was going to be, it was not going to be a social revolution, and he had settled that personally.
Doctrine is abstract. What ordinary people experienced was a change in the room they had sat in every week of their lives.
The service was now in their own language, and they were expected to follow it rather than watch it. Congregational singing became central in the Lutheran lands, with hymns written to be memorable and printed cheaply, and it did more to spread the theology among people who could not read than any treatise.
The building itself changed, and in places violently. Images, statues, altarpieces, relics and stained glass were removed, whitewashed over or smashed, on the argument that they were idols and that the money spent on them belonged to the poor. Waves of image breaking swept Zurich, the Netherlands, Scotland and England, and a large share of medieval European religious art was destroyed by Christians in the sixteenth and seventeenth centuries.
Clergy married, which reallocated a great deal of property and created a new social type, the pastor's household, that produced a strikingly large share of the following centuries' scholars and scientists. Monasteries were dissolved in Protestant territories and their lands went mostly to rulers and to the gentry who bought them, which bought those buyers permanently into the settlement.
And the Catholic side reformed too, which is easy to forget. A council meeting on and off from 1545 to 1563 tightened discipline, required bishops to reside in their dioceses, established seminaries to train priests properly, defined doctrine against the Protestant positions, and reformed the indulgence system that had started the argument. New orders, above all the Jesuits, built schools across Europe and beyond that were widely regarded as the best available.
After decades of manoeuvre and a short war, the empire arrived in 1555 at a formula: each prince would determine the religion of his own territory, and subjects who disagreed could emigrate.
It is worth being clear about what that is. It is not toleration. It is the opposite: the principle that a state has one religion and that dissenters leave. It covered only Catholics and Lutherans, excluding Calvinists, which is one of the reasons chapter twenty eight happens.
What it did establish was that religious uniformity across Europe was over, and that rulers, not the church, would decide the question. The confessional state, in which the sovereign defines orthodoxy, polices it, and uses it to bind subjects to himself, is the sixteenth century's real invention, and it is at least as important as anything in the theology.
Firm: the texts and the finances. We have the theses, the treatises, the bull, the diet's records, the Fugger accounts and the print runs. The indulgence transaction is documented in detail.
Soft: the door. The famous nailing at Wittenberg is first mentioned by a colleague after Luther's death, and Luther never described it. Posting notices on the church door was normal university practice, so it is plausible and it is not attested at the time.
Soft: several of the famous quotations, as noted.
One thing that must be stated rather than left out. In 1543 Luther published a treatise on the Jews recommending that their synagogues be burned, their houses destroyed, their books confiscated and their teachers forbidden to teach on pain of death. It was reprinted by the Nazi regime four centuries later. Historians differ on how much continuity there is between medieval religious hostility and modern racial antisemitism, and it is not a defence to say that his views were common in his time; they were also more violent than many of his contemporaries, and they came from the most widely read author in Europe.
Contested: the consequences. Claims that the Reformation produced capitalism, individualism, modern science or religious liberty are all in circulation, and all are argued about. The liberty claim is the weakest, since none of the principal reformers believed in it and several enforced conformity as harshly as anyone. What can be said with confidence is that Europe acquired permanent, organised religious division, that this division was fought over for a hundred and thirty years, and that religious liberty eventually emerged not as anybody's ideal but as the only arrangement the survivors could live with.