Real Dictionary

preach
verb: When a member of the clergy preaches a sermon, he or she gives a talk on a religious or moral subject during a religious service.
verb: When people preach a belief or a course of action, they try to persuade other people to accept the belief or to take the course of action.

preach in American English
to expound or proclaim by preaching
to speak in public on religious matters; give a sermon
intransitive verb: to deliver a sermon
transitive verb: to proclaim or make known by sermon (the gospel, good tidings, etc.)

preach in British English
verb: to make known (religious truth) or give religious or moral instruction or exhortation in (sermons)