I would also keep in mind the fact that other followers of the old god - Bane, I guess - probably won't be happy about the player defecting. Most priests are commoners who can't cast spells, so those who can would be seen as especially favored by the god. For a "chosen one" to turn their back on the religion could be theologically huge - even moreso if the character is level 6 or higher. Think of a Pope or a Sephardi Chief Rabbi or a Dalai Lama suddenly abandoning their old ways. You'd have shock and disbelief, and not a few parties interested in keeping the event a secret - by any means necessary.