verb
preaches; preached; preaching
to make a speech about religion in a church or other public place :to deliver a sermon [no obj]
to write or speak about (something) in an approving way :to say that (something) is good or necessary [+ obj]
[no obj] :to write or speak in an annoying way about the right way to behave
preach to the choir
chiefly US or preach to the converted
to speak for or against something to people who already agree with your opinions