From Longman Dictionary of Contemporary Englishpreach to the converted/choirpreach to the converted/choirEXPLAINto talk about what you think is right or important to people who already have the same opinions as you → preach
Examples from the Corpus
preach to the converted/choir• But they will still usually be only the party faithful, so he will find himself preaching to the converted.• To some extent this means preaching to the converted.• Not only was he preaching to the choir, he was talking to tax-cutting evangelists.• You're preaching to the converted in us, but you've got to get at everyone else.• He is preaching to the choir of religious-right Protestants and conservative Catholics whose votes should already be locked up.• As Chesterton was preaching to the converted there was no chance of this, so he avoided prosecution. 