If we're going to have NPC given sermons, I would like to see that the system doesn't come up exactly the same as infamous X - those who know what I'm talking about know what I'm talking about. I would suggest a system of frequent sermons in the temple, e.g. in the dawn, noon or so on. This could also differ from deity to deity. (ed) A system that consists only of the sermon is BORING and everyone can ignore it and so, so I'd go for a system that has a bit more depth and variety from deity to deity.
My suggestion: Services, consisting of sermon + some stuff.
Some lists for services (where sermons are held). One of the services is the "main service" of the day, marked with an asterisk (*).
Taniel: at noon service of the *Sun; at twilight, service of the Conquering Sun
Evren: at dawn, the service of the *Refreshing Dew; at noon, the service of the Sun
Gwen: at 15.00 service of Warm Love, at 22.00 service of *Nightly Love
Zhakrin: at dawn, the service of Balance in Light, at twilight: the service of Balance in Darkness; midnight the service of *Hidden Balance
Asral: at 06.00 service of Ignition, at 15.00 service of *Burning Fire; at 21.00 the service of Calm Fire
Lilith: three services of *Chaos spread around the day randomly
Sathonys: at twilight the service of Conquering Darkness, at midnight the service of the *Dark Kingdom, at 03.00 the service of Overpowerful Darkness
The services would differ from each other and consists of several prayers, sermon and perhaps a creed or so, of course differing from deity to deity. Of course, Lilithian service would always be random
After I finish my current project, I can make all the abovementioned (Autumn).