This is an offshoot from an excellent post on the Stocking The Dungeon blog. Up until I got the gray box, I never really bothered with detailing my gods while playing D&D. Clerics just followed God or some evil force. It wasn't important. Religion wasn't a huge factor in the early Known World products that I had and used (as presented in the Isle of Dread, which was the basis for my first few campaigns).
Once I bought the gray box, though, that all changed. Religion was IMPORTANT. Lots of gods were introduced, and they had friends and enemies, and they affected the world. While this isn't really a big surprise for most of us, I was only 15 at the time and it clicked for me that I really should make the gods more important in my games.
Every since then, I've always made a point of having religion matter. Even if it's just the names of artifacts or the priests giving blessings. I was a huge fan of the Forgotten Realms for many years, so much of this was done for me, and made my life a lot easier. Faiths and Avatars was a great supplement that I got a lot of use out of. The other religion books were also well-received, but I've never owned them.
Thinking about it, I wonder how many other gamers had this happen to their games?