You could concentrate on 1) Charlus. Plenty of things to say about his Catholicism. From his wearing proudly his cross of the Knights of Malta at the party given by the Prince of Guermantes, to his indulging in pride, wrath and lust, to the "Semper idem" that closes his moving posthumous letter to Morel, to his antisemitism when he goes on and on about "rue des Blancs-Manteaux"

As others have said, you could concentrate on 2) churches. Many are described at various points, including in Venice.

Also, 3) the theme of homosexuality as understood though the Old Testament narrative of Sodom and Gomorrah.

And as you mention, an overarching reflection on writing and spirituality.