to deliver a religious address such as a sermon to an assembly of people, especially in a church, or to publicly teach some religious belief or message;
e.g. He joined the clergy and went to preach in a small town.
(verb)
to earnestly urge or advocate for something, especially accepting or abandoning a course of action or belief/idea, often in a self-righteous, pompous and tiresome way;
e.g. She likes to preach at them to avoid generating so much waste.