"The time has come," the Walrus said,
"To talk of many things:
Of shoes--and ships--and sealing-wax--
Of cabbages--and kings--
And why the sea is boiling hot--
And whether pigs have wings.
The Walrus and the Carpenter ~ Lewis Carroll
As I have looked over my work from the last few weeks, I find myself drawn back into the questions of how does my little world work and what rites are performed in it whereby the people of it might somehow participate in the workings of that world. It is a matter that has had me looking into my books and research on the fundamentals of ritual magic and superstition. (J.G. Frazer's The Golden Bough is an excellent place to start. Go with the multi-volume edition. There's a lot more interesting detail there.)
If we accept the premise that rituals are humanity's attempt to participate in the turning of the cosmos, it leads one to wonder how do demigods fit into the picture and beings of magic (such as the true dragons of Evandar). It is a thorny question to ponder because one finds themselves looking again at their own personal rituals and how their spiritual life unfurls. Within the world of Evandar, there is no division between spirituality and life. Are there atheists in this world? Yes, but they are in the minority and a relatively 'modern' phenomenon.
It is hard to build a religion from the bottom up. Many people start out with the habits and behaviors, including linguistic elements, and develop their theological elements as per what is needed in a given scene. While it is possible to build a believable system in this fashion, it becomes harder to track why people do what they do. It requires a great deal of cross reference within the universe to ensure that element A remains distinguishable as element A consistently through the various story arcs that develop.
Top down building of religion, however, is a bit easier because the reasoning of why different practices develop is placed within the spiritual/philosophical context that it has arisen from. A god of storms will remain a god of storms in the different contexts you place your characters in. The rituals that are devised for them and their superstitions fall in some manner of logical order because they have the central reference point of said deity. There is no muddying of the waters with a deity being portrayed differently within the context of the same cultus for no apparent reasons. Person A who follows said deity will have points in common with person B who follows the same deity, regardless for what regions they are from.
And the development of regional variations in modes of worship and iconography becomes more organized because there are central themes that hold true through the different regions. The god of storms will remain a god of storms through out the whole of his body of worshipers, though regions may ascribe additional traits to him that reflect the unique needs of that region. This may sound odd but when we consider the religions of antiquity, we start with the pantheon and then work our way down into daily rites and superstitions.
In all likelihood, religions develop from the bottom up. As personal gnosis is shared a recognizable set of traits are revealed of said deity and this becomes the publicly accepted image of said deity. Just as person gnosis is verified and shared, thereby becoming communal knowledge, personal actions of worship are shared and a corpus of approved public rites develops from the common themes. Add into this the influence that powerful people have upon such things and one can discover how religions arise and develop within a given region. Throw on top of all this the interplay between different cultural groups and what interplay there is between the deities and one can map out the evolution of a religion from a small personal belief system to a complex state religion that incorporates elements from cultures it interacts with. (Ancient Rome, I am looking at you.)
It is an enormous amount of work to build a religion. It is tempting to say that the people believe in deity ABC and do XYZ. And to just leave it there. It is, however, far more fulfilling for the readers to find traces of the deity and their worship littered through the story in logical places. It makes for a more immersive experience. It also gives you, the author, the opportunity to explore philosophical questions that you may have always wondered about. It also allows you the chance to give commentary upon the philosophical questions and ethical quandaries that arise within in our world. (The late Terry Pratchett was a genius at doing this.)
Can you have a character that never mentions anything of their faith? Yes. It is, however, harder to portray the challenges that arise within one's dealings with the world that are subtler then hammering a nail and physical violence. Give your characters the chance to tackle the big questions. Explore what is right and what is wrong within your world. See what happens when young ideology conflicts with old dogmatism. Your story will benefit from it. And you may surprise yourself with the subtly of how you handle these things.