The Collaborative International Dictionary of English v.0.48:

Sermonic \Ser*mon"ic\, Sermonical \Ser*mon"ic*al\, a.
   Like, or appropriate to, a sermon; grave and didactic. [R.]
   "Conversation . . . satirical or sermonic." --Prof. Wilson.
   "Sermonical style." --V. Knox.
   [1913 Webster]