As i have studied a bit of theology (Eastern Orthodox mostly) I shall write a couple of opinions.
At first glance, not an easy task to write a Christian theme in a Fantasy world. But much more easy than, for example, writing an Islamic or Atheist fantasy, because the first adheres to strict monotheism, the other to strict materialism (although Atheism goes well for science-fiction, but this is a different discussion).
I first thought to myself, what is in fact the narrative essence of Christianity ? In my opinion a large part of fantasy literature is in fact filled with Christian themes. I shall stop at five that are most common:
1) That there is an age long cosmic struggle of Good vs Evil, and on the side of the good you have an all loving God and on the other side its adversary (Satan), and eventually good will triumph over evil.
2) That both God and Satan have various supernatural helpers, angels, respectively demons, but in other worlds these could be classed as lesser Gods; there would be absolutely nothing wrong with that as real-world Christians do worship and pray to angels and saints (and they have consecrated states, images, etc) just like polytheists worship lesser Gods.
3) That mankind has committed sin, the world and the Universe itself is tainted with it and thus mankind needs to struggle to release itself from the sin. Sin is the cause of death, suffering, all evil that happens in this world.
4) That there is an afterlife and a punishment / reward is given according to the deeds that one has committed in its life. That the basic commands are to love one another as God loves the world.
5) Strong morals. Enhance on issues such as family life, love, friendship, honor, forgiveness.
I did not put on this list the sacrifice of Christ and I shall explain why.
As far as i know, from a theological point of view, the sacrifice of Christ was specifically made for mankind, for ”this” Earth. The issue, surprisingly, was discussed to some extant by Church Fathers. It was not uncommon - back in those days, people believed there where other people on the Moon and on the Sun. Despite varying opinions among the Fathers, we cannot possibly know how the
divine pronoia manifested itself in other created worlds.
Some writers had adopted an
Earth-centric view, assuming that the
divine pronoia did not manifest in other worlds at all. In this category are the stories in which a person from Earth comes to visit and convert an Alien species, or in which an Alien species is converted to Christianity from whatever (false) belief or non-belief they initially had.
Other writers had adopted an
Earth-like view, in which they assumed Christ has made a sacrifice there but under another name. Here is the story of Aslan from Narnia.
Some authors went for the idea that there might be some worlds in the Universe that did not experience the Fall. I found it highly problematic from a theological point of view. We can presume correctly, that if the Universe as it is, is filled with plenty of imperfections and natural evil (meteors, dying stars, black holes, etc), than all material living beings inside it are subjected to the same issues as Man (death, sin, etc). Because the Fall of Man meant that Man lived no longer in the presence of God, but in this Universe, made to be neither good or bad, so that each individual will experience death, could see good and evil and eventually choose its own path. If there where created beings who did not Fall into Sin, they cannot - and could not- live within the boundaries of this particular Universe but somewhere else.
Personally, in my novel I went for a complete non-Earth like approach, going straight for the 5 points i mentioned above. I found highly unlikely from a theological point of view the idea of
divine pronoia not acting on other worlds as i find also highly difficult the issue of Christ sacrificing itself for another world.
English is not my first language, but i hope readers will get the ideas of this rant