How did the Catholic Church become corrupt?

If there was one thing that made the Catholic Church corrupt to its core, for centuries to come, it was the creation of the document known as the Donation of Constantine. The Donation of Constantine was an eighth-century forgery purporting to be from Emperor Constantine in the fourth century, bestowing huge ecclesiastical and political authority upon Pope Sylvester and his successors. Pope Stephen II took his document to Pepin, king of the Franks, in 755, asking for help so that lands then held by the Lombards be "restored" to St. Peter himself, because four centuries earlier Constantine had granted them to Sylvester. In the grand scheme of things, this reflects on Stephen and his forger, probably a papal official called Christophorous, not on the Church as a whole, and there is no suggestion that subsequent popes actually knew the Donation to be a forgery.

The Donation of Constantine changed the papacy, making it a prize to be fought over or bought, and therefore changed the Church itself. The Papal States won by Stephen and subsequently consolidated and expanded, helped make the pope the richest man in Europe. During long periods of the Middle Ages, simony, murder and corruption became common currency at the highest levels of the Catholic Church. Popes who bought their way into office did not stop with living in incredible luxury, but openly kept concubines or mistresses, and provided for the 'nephews' who resulted from these unions, often promoting them to the positions of bishop or cardinal while still in their teens.