Other than putting words down on paper, the task I spend the most time on while writing is deciding upon the order in which those words appear. The order of sentences, the order of paragraphs, the order of scenes. Should I reveal a nugget of information now or wait until later? Argggg! I tweak constantly, rearranging everything over and over until I feel I’ve hit the sweet spot. My hardest task, though, is deciding when and where I should drop in world-building info.
The commonly accepted rule is that authors shouldn’t reveal world-building facts until the reader needs to know them. Otherwise we run the risk of slowing the pace of the story and possibly boring the reader. Like most rules in writing, however, I’ve found this rule to be more of a guideline, because it’s possible to take this concept too far.
For one thing, common sense suggests writers shouldn’t always wait until the last minute to deliver world-building info. If the reader keeps learning necessary facts right before those facts become important to the story, he’ll spot the pattern and grow annoyed. It’s often better to drop in these bits of information well in advance of when they’re needed.
I can also imagine instances in which a writer will deliberately want plenty of separation between the delivery of info and when it’s actually needed. Consider mystery stories, for example. The writer needs to drop in facts/clues about the surrounding world so the reader has a legitimate chance of figuring out the mystery before the big reveal, but the writer usually wants to place them early in the story so those facts aren't foremost in the reader’s mind as the climax approaches. Otherwise it might be too easy for the reader to solve out the mystery.
Or how about when the author needs to foreshadow a future event? In many cases, it’s better to foreshadow well in advance, in order to give the reader time to build up anxiety over what might happen.
Heck, sometimes world-building info isn’t always needed for the story. Sometimes the author adds this kind of information solely for the purpose of the reader’s entertainment. This often occurs in fantasy or science fiction, where dropping in little details about how things work in this world are part of the draw. If the author followed the aforementioned rule, he’d never be allowed to add this info.
Of course, just because you can drop in bits of info early doesn’t mean you’re relieved of the responsibility of coming up with a valid reason for tossing it in there. No matter how entertaining you might find the religious system in your newly created world, the reader will not be pleased if you dump it on him for no reason. As a writer, it’s your job to dream up situations that justify the inclusion of this information. The reader may not need to know the information yet, but they don't need to know that.