I mentioned in my earlier post that both Magic-Users and Clerics have spell books, in contrast to the later distinction between Magic-Users memorizing spells from their books and Clerics praying to their god. This distinction seems to have come in via Greyhawk, which according to Rob Kuntz in a thread on the OD&D 74 forums is the way he and Gary actually ran D&D, even before the publication of the LBBs. In Greyhawk, we first see the rules for minimum and maximum number of M-U spells knowable, based on Intelligence, and a comment that Clerics have access to all the spells on the Cleric lists, because their spells come directly from their higher power.
So, since pre-Greyhawk Clerics don't seem all that different from M-Us, what is the difference? Clerics don't get spells at 1st level, for one, and can't cast as many spells per adventure as an M-U (about half as many.) Shorter spell lists, for another, and not as many options: Clerics in the LBBs can't cast spells that cause damage, but they can cast healing spells, which M-Us don't have access to.
But what about the reversed spells?
Reversed Cleric spells are said to be available only to evil Clerics, also called "anti-Clerics"; the one exception is the Finger of Death spell, which ordinary Clerics can cast, but only in dire circumstances; they may potentially lose their Cleric status, becoming anti-Clerics, if they misuse the Finger of Death. In exchange for the ability to harm, anti-Clerics lose the ability to affect undead (in contrast to later editions, where evil Clerics can command undead.)
But otherwise, Cleric spells don't seem to depend directly on devotion to a god in order to function. There's no mention (yet) of Clerics ever losing their spells because of transgressions; the turn undead ability seems to depend on faith, but the spells just seem to be knowledge, although it seems to be a different tradition of spell-casting, opposed in some ways to the Magic-User tradition, based on this comment about changing classes: "In any event Magic-Users cannot become Clerics and vice-versa." Once you learn a particular paradigm for working magic, you can't change, nor can the gods take it away from you (except maybe by sending a level-draining spirit or simply killing you.)
Anti-Clerics seem to be based more on medieval concepts of Satanists than on devotees of evil gods. These are demon-worshipers who make bargains with the infernal powers. Since "Anti-Cleric" is too distinctively a part of the LBB product identity and not in the d20 SRD at all, what I think I will do for Liber Zero is call them Heretics, introducing Heresiarch as the evil equivalent of Patriarch (although players can still opt for Evil High Priest, if that's what they prefer their characters to be called.) Perhaps a good house rule is to allow different demons or unholy things to grant some kind of replacement power for the turn undead ability -- if the heretic meets their new master's stringent demands. Other heretics who have turned away from the "true faith" but haven't sworn allegiance to an infernal power get no replacement powers, just unrestrained use of reversed spells.
Hmmm. This now makes it possible to have atheist priests, or priests of dead gods.