A very simple, lucrative, massively corrupt and corrupting example of the con was the selling of indulgencies (get to heaven quick promises) by the Catholic church. In the end, the practice led to the reformation of course, but it still exists today in a moderated form.
.........the later Middle Ages saw the growth of considerable abuses. Greedy commissaries sought to extract the maximum amount of money for each indulgence.[40] Professional "pardoners"[5] (quaestores in Latin) - who were sent to collect alms for a specific project - practiced the unrestricted sale of indulgences. Many of these quaestores exceeded Church teachings, whether in avarice or ignorant zeal, and promised impossible rewards like salvation from eternal damnation in return for money.[38] With the permission of the Church, indulgences also became a way for Catholic rulers to fund expensive projects, such as Crusades and cathedrals, by keeping a significant portion of the money raised from indulgences in their lands.[38] There was a tendency to forge documents declaring that indulgences had been granted.[38] Indulgences grew to extraordinary magnitude, in terms of longevity and breadth of forgiveness.
Indulgence - Wikipedia
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