Preaching to the choir means that you are presenting your opinion or argument to a person or group of people who already agree with your opinion or argument. The idiom preaching to the choir first appeared in the 1970s in the United States and is a variant of an older idiom, preaching to the converted, that originated in British English in the 1850s. Both idioms are expressions that address wasting one’s time arguing with people who agree with you.

Source: Grammarist