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preach to the converted

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]

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The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.