to preach to the choir

[Tourmaline]

Hello!

Today, while I was reading English sentences, I saw this expression.
"Preach to the choir."

I thought the meaning was similar with this expression
"to teach a fish how to swim."

But when I found some examples in google, I realized that the meaning is different.

Question:
1. Is the meaning of the expression 'to teach something to those who is well aware of it'?
When I write 'well aware of', it means just a certain fact, not the depth of the knowledge - something like between an amateur and a professional.

2. If I am right about the meaning, in reality, is the expression somewhat rude/showing anger or you can just say it when you say something like "I already know it."?

I hope I could make my queation correctly;;
Thank you!

 

[entangledbank]

I understand it as referring to beliefs, not abilities. 'Preaching to the choir' and 'preaching to the converted' mean wasting your time trying to persuade people who already believe what you're saying - the readers of that magazine, or the people attending that political convention, aren't the ones who need to be persuaded, as they already agree with you.

We have another colourful expression for trying to tell someone how to do something when they already know well how to: 'teaching your grandmother to suck eggs'.

 

[Chasint]

Hello!

Today, while I was reading English sentences, I saw this expression.
"Preach to the choir."

...

It would help if you could say what these sentences were and where they came from. I am sure that context would help a great deal.

 

[MirandaEscobedo]

'Preach to the choir', like 'preaching to the converted' does, indeed, mean wasting your time by trying to convince those who are already convinced on the matter. I am not aware of any big distinction between beliefs and abilities. Another expression in this series, 'teaching your grandmother to suck eggs' is, like the one you mention, 'teaching a fish to swim', about abilities. They all mean that your efforts are redundant.

Context could make a difference. Is it a third party giving advice ('Don't waste your time ...') or is it a second party replying ('You don't need to tell ME that!').