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