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. As AD&D™ games depend on participant input for their character, the detailing of deities and those who serve them is strictly a part of the role playing aspect of the game. Must all evil characters sound sinister? Does an elf have to be flighty? Need a ranger be lugubrious? Actually, the game system tells you what is necessary for a campaign, but how the campaign is role-played is strictly up to the DM and players.
Now, the two things I'll point out here are this. (1) This fundamentally contradicts the text he wrote in the Deities & Demigods book itself about what a core, indispensable element it was to AD&D. (He goes on in this article to say that he doesn't use the DDG in his own campaign, except in cases of transference to other worlds.) And (2) Note that his first supporting reference to the traditions of fantasy literature is -- as always! -- Tolkien. Only secondarily does he add in examples from Conan, Fafhrd, Harold Shea, etc.
As is often the case in these fundamentalist-text games, I'd be happy to claim allegiance to this position that "I do not agree that it [religion] needs be a significant part of the campaign", if it weren't for the awareness that you can find basically the exact opposite (regarding Clerics, etc.) in many other places in his writing.