Now, clerics in D&D are not priests. They’re actually holy warriors who are given miraculous powers by their “deity”. I say “deity” because this is somewhere the deep roots of D&D in mediaeval wargaming rise to just below the surface. Far from being the modern, or ancient, fantasy vision of a world full of gods and spirits, the core books of both OD&D and AD&D are very much a Christian view. Clerics are followers of Christ without the game saying so. In OD&D, the cleric’s “holy symbol” was specifically a cross. AD&D moved away from that slightly but the broad vision of the class is the Christian vampire hunter.
The practical effect of this is that God is literally everywhere - and feels like he’s everywhere - and that is one thing that makes playing a non-Christian cleric unsatisfactory.
Clerics use the same “theory of spells” as magic-users - the energy of the spell is implanted in the character’s mind and is released when needed. The difference is that instead of spell-books, the cleric’s deity gifts the energy requested, either indirectly or directly and in person.
The rules apply no restrictions on where this gifting takes place. They specify that the cleric gains them by “supplication” but, in keeping with the assumption of an omni-present god, the 15 minutes-per-level of praying or whatever form this supplication takes can be done anywhere and at any time.
This is clearly a major advantage to the adventuring cleric as compared to the adventuring magic-user struggling to find ever more secure (and un-heroic) ways of transporting their most valuable possessions across open wilderness or even into deep oceans or planes of living fire. That would perhaps be acceptable as just something that magic-users have to live with in return for their much wider range of spells and, indeed, two entire extra levels of spells. I’m not sure where that argument leaves illusionists, though. But there is a knock-on effect of this al-fresco worship which is more subtle but also much more deadly to the role-playing experience.
Because there is no requirements about place of worship, the cleric character need never enter such a place. A “high priest” can happily reach that position without the player ever having had to deal with their religious organisation, enter a temple or perform any overt act of religious ritual. The cleric trains under another cleric and, similarly, there’s no need for that training to take place in anywhere more religious than an Inn.
Similarly, there are no real rules about ritual cleansing or purification, since there is nowhere the cleric has to worry about entering while “polluted”.
All in all, this aspect of the cleric’s spell-gaining mechanic is one of the main reasons that AD&D and OD&D have a fairly notorious lack of colour. There’s pretty well no depth to the relation between a cleric and their deity, with the possible exception that many players dream of one day taking on their god, or some god at least, in combat.
For many DMs it can be hard to find experienced players even interested in playing a cleric as there’s little difference between them, and their spell mechanics means that they all cast the same sorts of spells. Once you’ve played one cleric, you’ve played them all. The odd magic-user here and there will fail to “know” fireball or even magic missile and regardless of that their spell lists will be dominated by what spells they have found while exploring. Clerics are heal-bots not just because the cure spells are fantastically useful, but because there’s no cleric anywhere who can’t cast them.
I’m going to skip over the various attempts to make a cleric’s choice of deity useful in AD&D as they were all pretty feeble. Instead I am going to discuss a cure for both this ill and the question of magic-users’ books.
