To sermonize is to lecture someone in the style of a minister delivering a sermon. The best teachers don't sermonize; they talk to students in a sincere and straightforward way.
If you tell someone at length what they should do, especially if there's plenty of judgment and moral superiority in your speech, you sermonize. This verb can also be used simply to mean "give a sermon," so you could also say that you like church because your minister sermonizes so well. This was the earliest meaning of sermonize, which comes from the Latin sermo, "speech, conversation, or talk."