One of the classic methods for converting someone to a different religion in pre-4E versions of the game was to cast an atonement spell. Pathfinder's version of the spell doesn't explicitly state that you can use it to switch religions, but rather alignments. I think that a reasonable GM would let you use the spell in that way, however. If you're using 4E, I'd say that you could perform some sort of ritual. Azagar's Book of Rituals, from Goodman Games, might have something along those lines.
If you're looking for something hard and fast, here's what I'd do as an on-the-spot idea at the table: have the character make a skill check depending on their method of persuasion. Diplomacy, Intimidate, or Knowledge (religion)/Religion could all be appropriate. Set the DC at 10 + the target's hit dice + the target's Wisdom modifier or use the moderate/hard DCs appropriate to the character's level. Require the character to succeed at a number of such checks equal to the combined total of the target's Wisdom and Charisma scores (Wisdom to represent spiritual fortitude and commitment to one's current path, Charisma to represent their zeal and passion for their current faith). You could also rule that certain actions result in automatic successes, such as the character casting divine spells on the target's behalf.
If you've got some time to think it through, you could make it more complex and nuanced... But I think it's the basis for something I'd come up with if I felt the need to design mechanics to support the idea.
If you're looking for something hard and fast, here's what I'd do as an on-the-spot idea at the table: have the character make a skill check depending on their method of persuasion. Diplomacy, Intimidate, or Knowledge (religion)/Religion could all be appropriate. Set the DC at 10 + the target's hit dice + the target's Wisdom modifier or use the moderate/hard DCs appropriate to the character's level. Require the character to succeed at a number of such checks equal to the combined total of the target's Wisdom and Charisma scores (Wisdom to represent spiritual fortitude and commitment to one's current path, Charisma to represent their zeal and passion for their current faith). You could also rule that certain actions result in automatic successes, such as the character casting divine spells on the target's behalf.
If you've got some time to think it through, you could make it more complex and nuanced... But I think it's the basis for something I'd come up with if I felt the need to design mechanics to support the idea.
