A post in which David fails to offend everybody reading. If somehow you become offended in the course of your reading, the gods have given us the comment feature. In any case, I'm not offended, so let's get on with it.
As a Writer, I'm not concerned with whether a character's beliefs are True, but with how those beliefs shape the character. On a larger scale, when I'm speculating a religion, the intricacies of its theology are not important so much as the effect that religion has on the characters in the story, to wit:
The character does (or refrains from doing) X because her religion says so. May involve much wrassling with conscience and character development. I prefer when the "because her religion says so" complements a more substantial reason for doing or not doing something than when it's given as the only reason. For instance, the character helps the oppressed because she's nice and because her religion says so.
Although a character whose entire motivation is built on religion can be interesting if they lose their faith and have to cobble together a working motivation for themselves out of whatever is closest to hand.
The Religious Zealot as the Villain is, of course, a stock character. I think it can be lazy, and it makes me lose a lot of respect for the Villain. There are so many other good reasons to be villainous. Nonetheless, it's fraught with pathos and can work really well in a redemption plot...or to explain why a good guy turns evil.
This is the character who thinks with his gut or, more artistically, his heart. As opposed to running everything through the "Does it Work?" filter, he runs it through the "How do I feel about this?" filter. When the gut is tied to a religion, the filter question becomes "How might [religion] feel about this?" This is the explanation for when that religious motivation is not internally consistent.
Most characters merely apply the Work filter more often than the Feel filter, or vice versa.
No Heinlein heroes need apply, and it's not to be confused with fuzzy logic or the more realistic kinds of idealism.
Religion as Culture
When the religion is practiced now because it's been practiced before. The character might be a member of the faith and have some mannerisms associated with it, or the character may be an outsider interacting with the distinctive scenery and other characters of faith. If I'm writing a story in small town Wherever, a local member of the clergy might be a fixture of the town. If I want my characters to have a shoot-out in a house of worship, it'd pack more punch if some character might be offended if they completely destroy it.
A book that does religion as culture really well is "The Mote In God's Eye" (Larry Niven & Jerry Pournelle), especially with the character of Horace Bury.
Philosophy as Religion
When religion gets scientific (but most people still call it religion). This is often where those more realistic kinds of idealism fit in. A popular feature of futuristic utopias.
There, now stop worrying and let your characters get on with their lives.
"This night shall go down in history as the greatest man-orgy of all time! And it all starts with this glass of wine!"
Even wise Socrates could not have predicted that the Ancient Greeks would get tired of man-orgies.
250 words? Yes
Book "Lived Too Long To Die" - writing the middle (~1/3 finished)
Book "Timpani the Ostrich Rancher" - pondering
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Reading - ?