Preaching to the choir is an English idiom that means a person is trying to convince or persuade another person or group to believe in or agree with something that they already believe in or agree with. Preaching to the chorus and preaching to the converted are similar idioms with the same meaning, but preaching to the choir is the most commonly used.
This particular idiom stems from the traditional church where the choir, or chorus, of a church stands behind the preacher singing hymns that express the belief of the church while the preacher delivers a sermon, hoping to reach the non-believers in the congregation. The preacher would not turn around and deliver the sermon to the choir rather than the congregation, as this would be considered a futile or pointless act. Thus, the idiom reflects the same principle.
Someone might say “you’re preaching to the choir” if you were offering a heated or passionate explanation of your social, political, or moral views and they already agreed with your position. This is a way of letting you know that there is or will be no debate to what you are saying and that they are in full agreement. They may also be implying that you can stop spending energy trying to convince them of your point because it is a point they’ve already concluded on their own.
Similarly, an individual may rant or vent his or her feelings on a subject and conclude with a statement such as “I know, I’m preaching to the choir.” This is an expression indicating that the person simply wanted to vent his feelings on a matter to someone he knew would understand and agree with him, rather than someone who would debate the matter or offer a different perspective.
Though the idiom is rarely used in conjunction with a verbal insult in social conversation, it could be an indicator that the speaker is wasting his time in conversation and should move on to another topic.