Claude Debussy, Préludes, Book 1: X. Profondement Calme (…The Absorbed Cathedral). Performed by Noriko Ogawa.
This is a solo piano piece with a binary, ABA1B1, form. Its tempo is marked as “profoundly calmâ€. Most of it is pianissimo (una corda, even), but there are some louder bits and accents. It’s basically, in my opinion, a chordal study. It reminds me a lot of Chopin’s Prelude in C Major (Op. 28, No. 20) and Chorale from the Nocturne (Op. 37, No. 1).
Like the Chopin Chorale that I just mentioned, I would almost consider this a religious work even though it’s not commonly considered so (as far as I know). I liked this one so much that I went out and bought the score for it and spent a lot of time reading through it.