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1.
Preaching to the choir means you are trying to make believers out of people who already believe, or convince people who are already convinced.
"We could start a new tradition, Birthdays without Boyfriends. It could work."
"Preachin' to the choir here baby."
by Kay May 15, 2004
 
2.
Extolling/defaming a cause to people convinced of it's greatness/horridness. Etymology is obviously from christian church choirs.
Playmate: Sex sells
Hugh H: Preaching to the choir.
by Kung-fu Jesus July 27, 2004