The Bishop of Meinz—one of the most powerful Bishop/princes (he had substantial political power) of Europe, with the express (though secret...now proven) collusion of the Pope himself—through the use of his bankers, “was[sic] the acts of a few corrupt individuals???”
These were the men behind the plenary indulgence that so PO’d Luther. Hardly an isolated individuals—rather at the heart of establishment of the 16th Century Roman church.
Luther was as upset about the unscrupulous manner in which the indulgence was preached (i.e., sold) as he was about the indulgence itself. And that was indeed the act of a few corrupt individuals, no matter how you slice it.
Nowhere, however, do we claim that ordination guarantees either prudence or moral probity. The idea that the Bishop of Mainz and the Pope acted corruptly is not at all a threat to Catholic theology or a contradiction of it.