When we say, “preaching to the choir,” we mean “preaching to the converted.” It suggests that a lot of energy is being squandered trying to convince people of something about which they are already convinced, which is why preaching to the choir, whom we assume to be among the converted, is considered a waste of time.

So why do it? Chesterton and Dale Ahlquist tell us in this article from the Bellarmine Foundation Forum.