Real-world Religions and the D&D Game

I'm currently undergoing research to find out about the real world religions, primarily those of the ancient world, but also such contemporary religions as Hinduism and Buddhism. Now the primary focus of my research is not the mythology of those faiths, but rather the day-to-day practices of those faiths. After all, if you play as a priest how often does mythology effect most adventures? Or if you go to the village priest for healing? It's not a complete non-factor, after all creatures of myth are a common opponent in D&D and the rivalries between divinities are often the focus of an adventure, but actual religion is often overlooked.

I hope to garner from my studies a greater understanding of how ancient religions worked and how it might work in D&D. A thing I have noticed is that religion is often summed up as "you worship this god, he has such-and-such a dogma, hates this and this god, etc.", as if each deity has it's own separate religion. There are such things as monolatrists, but even so this approach oversimplifies things. Yes, sometimes simplicity is good for the game, where overcomplexity would be bad, but I believe that a greater understanding of how religion worked for various cultures could in fact improve my games.