Anime has such a wide variety of subjects that it explores. There are two animes, Deathnote and Neon Genesis Evangelion that have both subtle and obvious allusions to religion/spirituality. They both posses theological elements, which can be interesting to write about.
Think about these questions:
How do they use various theological elements to add to the overall meaning of the anime?
What seem to be the most interesting symbols and why would you consider them as such?
Does one anime use the symbols in a more effective manner?
I was going to add as a suggestion. There's an older anime by Shinichirō Watanabe, the creator of Samurai Champloo, called Kids on the Slope. It doesn't suggest anything religious until the end. You find out that one of the kids becomes a Catholic Priest.It might be worth analyzing as well as the anime you've already mentioned. – Passerby3 weeks ago