Among the characters I'd as religion is negligible: It's mentioned that Cassie's family knows a member of the clergy, Jake and Rachel have Jewish roots, Loren later is a church crisis councilor, Eva sang in a church choir, and it's once mentioned but never seen that Jake's family says grace-None of which says to me that they have any sort of deeply felt and thought out religious believes themselves, could just be humoring their families.
I mean come on, teens with little to no religious feelings themselves blame or thank God for everything in their lives: these kids saving the freaking world don't talk about, or even mention God, except maybe once, but I may just be imagining that. KA was pretty solid about keeping whatever faith the kids may have had out of the series.
And between the Ellimist, Crayak, the Pemalites and the Arn, I really don't see her trying to keep creation the right of the divine either.
Now, the much more interesting part: SPACE RELIGION
The Hork-Bajir had mother sky & father deep as their creators/benefactors- came out as a charming little faith to me, like a mini Greek pantheon, and they were content with it till they learned the truth
The mention in 18 that the Leerans have a God, don't remember name, and since we never saw them again, I consider that free ground to make a fan fic about their spirituality.
Not really a lot of other references, but I have to make this call: In TAC Alloran tells Chapman to pray to "whatever primitive gods" he has. Now while it seems that the modern Andalite doesn't seem much on spirituality, they have been around for 65 million years or more (reference TEC, MM2) and I'm sure they had some sort of religion in there, and I wouldn't be surprised to hear that at least the Andalites find it very well developed, and probably a bit more subtle than create/control something & love or damn compared to the faiths little old humans have. Something to ponder.
And I guess I feel inclined to explain that spiritually I'm more of an adamant denier of the divine (I had my falling out with the atheist when I found they had a national org. I mean,WTF, why?) for my own personal affairs. I see a cultural and personal value in having faiths, just don't have any for myself