preach to the converted

preach to the converted idiom
Try to convince someone who is already convinced, as in Why tell me smoking is bad when I gave it up years ago? You're preaching to the converted. [Mid-1800s]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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