On 31 October 1517 Luther sent a letter to Albrecht of Brandenburg, archbishop of Mainz, objecting to the preaching of an indulgence in his territories, and enclosed a set of propositions for academic debate. The indulgence in question was being preached by the Dominican Johann Tetzel; half the proceeds were going to the building of St Peter's in Rome and half, unknown to those buying, to repaying the enormous debt Albrecht had incurred to acquire his own archbishopric.
The theses are in Latin and are a scholarly disputation rather than a manifesto. They do not deny purgatory, and they do not deny the pope's authority to remit penalties he himself has imposed. What they attack is the claim that a payment can release a soul from purgatory, the effect of that claim on ordinary people, and the confusion of penance with a transaction. The first thesis sets the tone: when Christ said repent, he meant the whole life of believers to be one of repentance, which is a statement about the interior life and not about money at all.
The famous nailing to the door of the Castle Church is uncertain. Posting a disputation on a church door was the ordinary way of advertising one, so it is entirely plausible, but Luther never mentions doing it, and the earliest report comes from Melanchthon after Luther's death. What is certain is what the printers did. Within weeks the theses had been translated, printed and circulated across Germany, and an academic exercise had become a public argument.