I'll take this one step further by noting that, prior to the introduction of Expert classes in D&D 3x, the vast majority of NPCs (to say nothing of priests specifically) in offical AD&D/D&D supplements were actually classed characters (e.g., Clerics, Magic Users, Fighters, etc).
Every city guard, serving wench, and barkeep had levels in something.
I personally never cared for this, but I'll be damned if there were rules for detailed non-classed NPCs in the core books prior to D&D 3x. The most in depth exploration of the concept that I can recall was the introduction of 0-Level characters in the much maligned Greyhawk Adventures hardcover (which, while clumsy, I still liked).
This is one area in which I think newer editions of D&D have vastly improved upon older editions (at least with regard to core rules).
Hmm, I kinda like the Normal Man rules of eg Moldvay B/X. However, I'm now giving PCs and major NPCs a hit point kicker of up to +10 hp, for some reason getting their hp into double figures greatly greatly helps with NM characters (the princess, the merchant) feeling 'real' and not just cardboard props.