The spirituality of Malacandra tells of a hierarchy of beings: eldila, Oyarsa, Maleldil, and The Old One - angels, archangels, Jesus and God being the Christian parallels. The tale is told of how Earth's Oyarsa fell, caused mischief in Heaven and on the planets, and was imprisoned on Earth.
And so it is that we find wicked Earth men arriving on Malacandra to spread their violent ways, bringing Elwin Ransom as some kind of sacrifice so they can plunder the planet's plentiful supply of gold. Ransom, on the other hand, having escaped his captors and discovered intelligent life on the planet, toys with the idea of being a kind of missionary to the planet... the irony only becoming apparent later on.
Ransom initially encounters, at a distance, the more or less humanoid seroni, who he assumes to be the only intelligent life forms on the planet, and also out to get him. However, it is with a hross that he first manages to make contact, discovering them also to be one of three species of intelligent Martians, the third being the pfifltriggi.
These three races represent three facets of human activity: the seroni are the scientists, the hrossa poets, and the pfifltriggi the builders. All three are untouched by sin, and work together in harmony, acknowledging their differences but none of them claiming superiority. All of which brings us back to the question of what it means to be made in God's image, and what it means to be human.
As science fiction, well, the narrative style doesn't compare to your modern space opera (and at 187 pages, neither does the word count!), reading like a slightly more poetic HG Wells. Lewis embarks on detailed descriptions of the Malacandrian landscape, flora and fauna, as well as the social and spiritual culture of the world he creates. Lewis also takes the unusual step for a sf novel of teaching us the alien language, at least some of the key terms. He manages to pull this off by introducing his protagonist as a philologist, which in itself may be slightly contrived, but makes the whole interaction with Martians feasible.
Of course there are huge flaws in the science, but Mars, and even space travel, seem to be reasonably accurately represented according to the knowledge available in 1938, when it was first published. It still works as a science-fantasy rather than a true science fiction, does some interesting things, and sets the mould for later Christian sf by the likes of Stephen Lawhead and Chris Walley.