When the speech delivered happens to be a sermon, the stakes go up even higher. There is a text, and a presence within the text, that wishes to be heard. The preacher must listen as well as speak, performing an act that is more complicated than solitary creation. There are a dozen things that must go right in order for communication to occur, and not all of them are under a speaker's control. Because language is a communal act, as much depends on those who listen as on those who speak.
Every word is a smoke signal sent up with great effort.
The fire must be very hot. The wood must be very green. The wet blanket must be lowered and lifted at just the right moments. But none of those is any guarantee. If no one is watching the sky, you might as well be roasting marshmallows.