Three hundred and forty years ago, a priest came up with the idea of praying that Lucifer would repent of his pride. After all, if Lucifer were to repent, the whole problem of evil would go away, and God's work would practically do itself. The Church fathers had no problem with monks and acolytes praying for the redemption of the Father of Lies, but there were less happy with a later development.
The successor to the order's founder decided merely praying for Lucifer's redemption was not enough. He proposed it was the duty of every monk in the order to prepare to make the case for repentence to the Infernal Father himself. Therefore, it followed, it was the duty of every member of the order to commit one mortal sin, and never seek absolution for it.
Generally, no one ever knew what this sin was for any other member of the Order. To speak about this sin was to run the risk of confessing it, and if it was confessed, it might be absolved. If this sin was ever absolved, the monk might be required to commit another moral sin in order to remain in the Order.
The rest of the faith objected to this theology, for reasons which, on later reflection, might have been considered obvious.