[preach] to the (choir|converted)

to attempt to make people accept ideas, opinions, etc. that they already agree with

The idiom "preach to the choir" originated from religious contexts, specifically within Christian churches. It is used to refer to the act of delivering a sermon or religious message to an audience that is already convinced or converted to the beliefs being preached. The phrase implies that the speaker is addressing individuals who already share their views or beliefs, making the message unnecessary or redundant.

The political candidate's speech at the rally was well-received, but some argued that it was preaching to the converted since the attendees already supported their campaign.
The professor's lecture on the benefits of exercise in a sports science class seemed like preaching to the choir, as the students were already knowledgeable on the topic.
The author's book on the importance of education received praise, but some critics felt that it was preaching to the choir, as its readers were likely already aware of the value of education.
The speaker passionately advocated for stricter gun control laws during the conference, but most of the attendees were already supporters, so it felt like preaching to the choir.
The environmental activist gave a powerful speech about the urgency of combating climate change, but the event was attended mostly by environmentalists, making it feel like preaching to the converted.