Preaching to the choir
"Preaching to the choir" or "preaching to the converted" is a figure of speech that refers to a situation when someone is making an argument to an individual or group of people who already agree with that argument.
Such a rhetorical strategy is meant to minimize dissent or further solidify a view. The range of responses to this, from fervent nodding ("So true, so true!") to disgruntled remarks ("Thank you for wasting my time, Captain Obvious!"), may all be expressed by commenting that that's preaching to the choir. The term implies that individuals who would be willing to sing in a church choir would be favorably inclined to whatever point of view the preacher was espousing.