Great thoughts, Jonathan!
I haven't written a lot of fantasy, and so far the bit that I've written tends to be either allegorical or without defined Christianity, but I've given this a lot of thought in sci-fi settings.  Because most of my sci-fi is futuristic, it's almost like creating a fantasy world in some ways.
I'm in very much the same boat with Phil.  I haven't been to church in years, much to my grief.  It has not been my decision.  But for that reason, I too tend to unconsciously leave church and rituals out.  In Firmament, Andi and the Doctor don't go to church, because there is none on the ship -- but it seems like it would be a good idea to have them meet with any other believers on the ship.
Or in Implant, there is no church or religious rituals because the main group is under excessive attack, basically like war time.  In fact, there's no mention of religion at all in the story.  It's presumed that the main characters are not Christians.
But these are things I should think about more.  If Christianity exists, whether in another world or another version of this world, then, as you say, it will have its rituals and traditions.
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What are your thoughts on Christianity in a secondary world?
I think it's wonderful when done right, tragic when done wrong, and not always necessary.
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Do you have Christianity in your secondary world?
In Learsi, not really, though there is a God as well as a Christ.  In The Ending, no.
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If so, do you prefer to write about our faith as it was under law or as it is now under grace? 
The current Learsi book is on the law and on the verge of being under grace, but I think either is interesting.
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Do you think it is easier to write about Christianity under law or under grace, or do both present a variety of challenges to overcome?
They both have different kinds of challenges.  Under law may be slightly on the harder side, as it's stricter, and probably darker.  Under grace, however, can become tricky when you have Christ characters, because that is tremendously hard to pull off.
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Have you read any Christian fantasy novels that portrayed Christianity in a secondary world? 
Not Christianity as such, though I've read allegorical (or "supposal") versions, like in Narnia and TiraNor.
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What did you think of these portrayals, and what were they like? 
In Narnia, Lewis does an incredible job, most of you are familiar with that.  He has a Christ figure who is sacrificed for the sins of another, which has always been a powerful example to me since I was a little kid.  
In TiraNor, the God is named Elshua, and is not a "character" really.  It's more like the real God.  Elshua is unseen, but empowers characters and speaks to their hearts, and very obviously guides the events of the world.  Elshua is spoken of often.  
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Did they portray Christianity as under law or as it is now under grace? 
In Narnia, both, though the majority of the books are grace, I suppose.
In TiraNor, as the books are retellings of Old Testament Bible stories, they are under law, in the sense that there is no church, no Savior, and Elsuha's will is given through Seers (prophets).
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Did you find them effective, and if so, why?
I found Narnia very effective, because of the way Aslan is portrayed.  Loving, but firm, truthful, kind, terrible, forgiving, wonderful.  It helped me see Jesus better.
TiraNor was also effective, the way that Elshua gives strength to JaRed at the moments where he needs it most, sometimes with just a whisper of hope.
I don't know if that answers your questions, I know you weren't really talking about allegorical Christianity.  But maybe those examples will give insight anyway?  Thanks for the great thoughts!
(and forgive me for the extremely long post... 

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