3rd edition, relics and rituals (Swords and Sorcery book/scarred lands).
Bit abbridged because it's sorta a long entry.
Enchantment (Compulsion) [Mind-Affecting], Level:Clr 9. Components: V, DF, Target: One creature, Duration : Permanent, Saving Throw: Will negates, Spell Resistance: Yes
A cleric may attempt to forcibly convert that person to the cleric's own religion. A will save negates the attempt, but the subject will be a little less resistant to such attempts in the future, cumulative -1 penalty to will save for each successfull save against this spell when cast by the same spellcaster (max -8). Same bonuses may be applied to a creature's will check, +4 For Good-> Evil God or vice verse, +2 for Neutral -> Good or Evil (but not vice verse), +4 for Law->Chaos (and vice verse), +2 for Neutral -> Law or Chaos (but not vice verse). Also, add their caster level for any divine spells to their save.
An additional -4 penalty is possible if the cleric is able to cast it subtly (requires 10 minutes of conversation)...
Successful casting doesn't change the alignment of the target, but each week after being converted, they shift one step closer to their new god's alignment.
Successful conversion implies the target will be well disposed to the cleric for 'showing them the light'. They can also continue to cast divine spells (and clerics can choose new domains as appropiate). Some classes, may loose their divine spells as a result of the alignment change (Paladin)...
Atonement may reverse the effects of convert, however atonement requires a truly repentant subject and those affect by convert are normally sincerely disposed to their new god. Usually requires a convert attempt by a cleric of the target's former god (does not recieve the alignment modifier or divine caster level bonus to saves in those cases).
Big bit of situational consideration (and also prolly bit powerful for a spell at 9th) is what makes me believe this is supposed to be an instaneous effect as opposed to a permanent effect (why bother with converting or atoning again if you can just dispel it?).