I've been listening to some online classes in preparation for seminary next year and one of the professors made an illuminating observation in the class I listened to yesterday.
He said that there seems to be a pattern throughout the history of the church that is particularly evident in the early church and around the time of the Reformation.
First, there is a synthesis between Christianity and paganism that captures the attention of the church. In the early church this was Origen and leading up to the Reformation it was Aquinas.
Then, because of the syncretism a heresy springs out of it. Because of Origen's flawed view of the Trinity, that Jesus derives His being from the Father, the heresy of Arianism not only sprang up but was accepted by the vast majority of the Roman Empire. Because of Aquinas' combination of Greek philosophy and Christian theology and elevation of natural reason to the same level as Scripture, this strengthened the Catholic belief that church tradition and scripture were equal, and what came out of that was someone and a situation like Tetzel who, on the authority of the Rome, sold indulgences (or salvation) to people - trading what is only the work of God for money.