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Phrase(s): preach to the choir [and] preach to the converted
Fig. to make one’s case primarily to one’s supporters; to make one’s case only to those people who are present or who are already friendly to the issues. • There is no need to convince us of the value of hard work. We already know that. You are just preaching to the choir. • Don’t waste your time telling us about the problem. That’s preaching to the choir. • Bob found himself preaching to the converted when he was telling Jane the advantages of living in the suburbs. She already hates city life.
McGrawhill's American Idioms And Phrasal Verbs