I've been critical of Chinatsu's designs in 3H but (perhaps unsurprisingly) one of the things I do appreciate are the Nabatean designs. They're very consistent and unified.
Did you notice how I said Nabatean design and not Church design? The Church itself doesn't actually have the most consistent aesthetic. Oh, they all have the Seiros symbol and gold trim and stuff. But I mean that Catherine, Alois, Gilbert, and generic monks/bishops don't actually have a lot of shared design elements. Generic reclassing colors for church units are red and white, but Catherine is the only church rep wearing red. None of the Nabateans do. You can't even say they all wear white, because Gilbert doesn't. If they didn't have the Seiros symbol, you'd be hard-pressed to say that they all came from the same place.
Let's start at the beginning.
Sothis (True) is a relatively plain design, but there's a few elements that stick out: her long tabard, her crown of lilies, the tassels dangling, and that diamond-shaped centerpiece that vaguely recalls a heart.
Sothis to Sothis: we once again have the central tabard (here combined with a tassel) and heart shapes. The hair ribbons and dangling bows recall tassels, as well. Also prominent are golden curlicues and stars.
Seiros herself echoes the central tabard and Sothis's chest binding with that belt arrangement. Her boots have the same loop twist on the knee as Sothis's anklets. There are lilies in her hair that match the original Sothis's. Notice how her bangs work with her headband to make a distinct heart-like shape. Gold loop patterning on her cape hem, similar to that of Sothis and Rhea, as well as stars on whatever that chest harness is.
Rhea, where it's all coming together. We've got everything here; hearts, tassels, and stars, plenty and prominent. Again, the central dress tabard. Even the same dangling star as loli Sothis's crown. Notice how Rhea's cape bears golden flowerlike loops that exactly match the ones on Sothis's asscape.
Generic female Bishop is actually the one exception when it comes to these design elements carrying over to the church in general; it echoes Rhea's silhouette (fitted dress with shawl and long tabard). Given the consistency here, I'd bet that Bishop was designed after Rhea and company rather than the other way around.
Seteth's design is plainer, but looking closely we can see the pieces reappearing. His circlet is set with a star, his cassock has golden curlicues that match Sothis and Rhea's, and his cape and belt bear tassels.
Flayn's hair clips are an exact match for the diamond-shaped centerpiece of Sothis's original crown. She has a central tassel instead of a central tabard, much like Sothis herself--she even has floppy bows on her shoulders just like Sothis. Once more, the golden loopy pattern is prominent.
There's also an interesting color progression here. There's no blue on the original Sothis, did you notice that? She's got pink instead. Sothis (True) is pink and white; Seiros is pink and white (her pink hairtie definitely sticks out in her martial gear); loli Sothis is pink, white, and blue; Rhea of the present is blue and white. (Seteth and Flayn are primarily blue--except, Flayn's reclass character color is pink.)
This is even the exact progression you see them with in the game itself. Sothis (True) is on the cover, Seiros is in the opening cutscene, loli Sothis is in the first chapter, and then Rhea is encountered. I couldn't tell you precisely what it's supposed to represent symbolically; maybe just how things change over time?
(artbook scans come from @FantomThief)
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