to talk about something with a group of people who already agree with you preach to the choirMany websites seem to only preach to the converted, but others attract all sorts of people.
to try to persuade people to believe things they already believe (usually in continuous tenses) There's no need to tell us about the benefits of recycling. You're preaching to the converted.
Try to convince someone who is already convinced, as in Why tell me smoking is bad when I gave it up years ago? You're preaching to the converted. [Mid-1800s]