Though the scaffolding that surrounds Nick Page's book on the Reformation – 'A Nearly Infallible History of the Reformation' – is very much tongue-in-cheek, the main structure is composed of a plethora of facts.
His explanation of what indulgences are is particularly helpful.
"And so a whole purgatory-avoidance industry grew up.
One of the chief purgatory-avoidance schemes was the saying of a Mass for the souls of the dead. People would leave money in their wills to pay for Masses to be said for them, and the reward was that you would get some time taken off your stay in purgatory.
But that was expensive, and only really an option for the wealthy.
So another scheme was created in which ordinary people could buy a voucher which would give them – or one of their relatives – time off from purgatory. These promo codes shortening your stay in purgatory were backed by the authority of none other than the Pope. And they were called indulgences.
The way they worked was that the Pope had a kind of bank of unused merit. You remember all those saints and martyrs who were fast tracked into heaven? Well, clearly they hadn't needed all the spiritual merit they'd accumulated on earth. So all their unused, spare merit, went into the Church's treasury, where it could be dished out by the Pope to any soul he deemed worthy. And, for a small fee, you could actually buy some of this merit in the form of an indulgence.
Indulgences had originated as a special bonus gift for anyone who signed up to the First Crusade, but such was the popularity that they were soon made available to cash customers. And it was not just your own stay in purgatory that could be shortened: the papal bull Salvator Noster (1476) extended this insurance to cover not just the living but also the dead. It acted retroactively.
A lot of people questioned the sale of indulgences; Luther was by no means the first, or the only person to object to this trade.
Earlier in 1517, for example, magistrates in the German city of Rostock strenuously opposed a three-month indulgence sale that had just been launched. What particularly miffed them was that they were already supporting 204 monks and nuns in their city and 182 working altars.
But the Pope needed money. Building new cathedrals and financing crusades did not come cheap. For both sinners seeking heaven, and for popes seeking finance, every little helped."