preach to the converted idiom (also preach to the choir)
向已皈依者宣教布道;劝别人相信他们已经相信的东西
to try to persuade people to believe things that they already believe
You needn't bother telling us how recycling helps the environment, because you're preaching to the converted. 你不必费口舌告诉我们回收利用对环境有什么好处，因为我们早就知道了。