This capable and knowledgeable individual suggests that data on the deities is insufficient for usefulness in an AD&D™ campaign. That religion, being so much a part of our real history, must likewise play a part in your campaign, J. R. R. Tolkien did not agree, for he wrote many pages without mention of religion. Most of the heroic fantasy and swords & sorcery books written do not feature any particular religious zeal on the part of their protagonists. Consider Conan, Fafhrd and Grey Mouser, Harold Shea, and the list goes on and on. I do not agree that it needs be a significant part of the campaign...
... Development of ideologies, rites, dogma, and so forth is purely a matter for the DM— with active participation of players, naturally. It is nothing which we desire to force upon players, nor will we. How a game is role-played is a matter of choice.
One unexpected thing here is seeing EGG turn to JRR Tolkein as the first-and-foremost authority of what belongs in a fantastic adventure. You can dig up a lot of quotes where EGG disavows being deeply influenced by Tolkein, and yet here when it suits his rhetorical purpose it's the first thing he thinks of.
Secondly, the advice here is at odds with what EGG wrote for an introduction to the DDG itself. There he offers it forth as an indispensible, necessary part of the core AD&D game. I guess I'll never entirely shake my 12-year-old naivete that publishing copy may be skewed in favor of whatever product is currently the target of sales efforts.
But thirdly in this case: I completely and strongly agree with the core analysis in this passage. When I think of precisely these authors, and precisely these characters -- personally I can't shake the impression that "Most of the heroic fantasy and swords & sorcery books written do not feature any particular religious zeal on the part of their protagonists."
I also "do not agree that it needs be a significant part of the campaign". The fit of the D&D cleric has always bothered me, and the campaign-building catastrophe that always befalls me as I try to fit it in, determine where clerics sit in the world power structure, detail a host of gods and churches before starting play, and try to fit together (1) Catholic-style crusading priests with (2) miraculous everyday powers, in (3) a paganistic pantheon of worship, is perhaps my biggest grief with D&D and all its traditions. As I've written before, I've finally expunged clerics from all the D&D that I personally develop (in OD&D, replacing clerics with Greyhawk's thieves). And here I'm rather stunned to find EGG one day arguing the same thing, when freed from the task of directly selling a specific product.