You are preaching to the choir
This idiom means that you don't have to say more things to another person because she or he already agrees with you.
En español sería algo equivalente a no desperdiciar el tiempo tratando de convencer a alguien que ya está totalmente de acuerdo.
preaching to the choir |
Example:
You: I know many governments are corrupted, because ...
Your friend: Yes buddy, you are preaching to the choir.
Se que muchos gobiernos son corruptos, porque ...
Si amigo, no desperdicies tu tiempo, porque estoy totalmente de acuerdo.