That is almost right. It is more like you are trying to convince someone of something that they already agree with. If you tried to convert your church choir to your religion, you are wasting your time because they already agree with you.

Hey Rodolfo, how are you? You are fairly correct! The idiom ''to preach to the choir'' (also ''preach to the converted'') is usually used to indicate that someone argues in favour of a viewpoint already held by one's audience. On average, it means ''to try to convince someone about something that they already support'' / ''to state one's opinion to those who are already most receptive to it.'' Please, have a look at the examples below for a better understanding: E.g.: ''There is no need to convince us of the value of hard work. We already know that. You are just preaching to the choir.'' E.g.: ''You're preaching to the choir here - we all have kids and understand how busy life can get.'' E.g.: ''Honestly, you're preaching to the choir, but I just don't have any money to donate.'' E.g.: ''I would ask you not preach to the choir here - I am fully aware of my duties!'' Hope that helps! Should you have any further questions/doubts regarding English grammar, do not hesitate to contact me. I shall be pleased to help.

Hi Rodolfo, this is commonly used in Australia meaning to complain or carry on about something to someone who already agrees with you or is in the same situation. Its more about agreement than knowledge :)