Fig.: to explain your ideas or beliefs to people who already agree with you; to speak eagerly in defense of a cause that needs no support

Don't waste your time trying to convince me: on this subject you are preaching to the choir. I totally agree with you and I will support you 100%

Non perdere tempo a cercare di convincermi: con me sfondi una porta aperta, perché sono perfettamente d'accordo con te e ti appoggerò al 100%

Don't lecture me on smoking being a bad habit. You're preaching to the converted, since I had to give it up years ago because of serious health problems.

Non farmi la predica sui danni del fumo, perché con me sfondi una porta aperta, visto che anni fa ho dovuto smettere di fumare per gravi problemi di salute

Later synonym of preach to the converted, like a minister who is preaching to his choir, who already believe