This is a very fun spell, and as a priest spell it is available much earlier (like, immediately) than its researched equivalents Charm and Enslave Mind.
Unlike the two researched spells, Apostasy only targets Sacred units... including priests. With a reach of 50, its range falls between Charm and Enslave mind (20 and 100, respectively). It has a much lesser chance of working than the other two spells, as magic resistance negates it "easily", but considering that the targeted units will probably be of higher value, this is not a major disadvantage -- I'd rather fail 10 times to enslave a Knight of the Chalice than fail 5 times to take over a militiaman. That said, sacred units do tend to have higher magic resistances to start with, so a Spell Focus would be an easy investment.