Here's a breakdown of the metaphor:
* Preaching: This refers to the act of delivering a passionate speech or argument, often about a religious or moral topic.
* Choir: This is a group of singers who already harmonize with each other. They share a common goal and sing the same melody.
Therefore, "preaching to the choir" suggests that you are speaking to a group that already "sings the same tune" as you – they already agree with what you have to say.
Example:
"The politician gave a fiery speech about the importance of education, but it was all preaching to the choir since she was addressing a room full of teachers and school administrators."
This implies that the politician's speech was unlikely to sway anyone's opinion since her audience already believed in the value of education.