The language peculiar to a people or to a district, community, or class
preach to the converted
to praise or speak to those who already agree with you
I was preaching to the converted when I told my friend about the advantages of the new car.
List of common idioms, their meanings and examples of their usage in sentences
An expression in the usage of a language that is peculiar to itself either in having a meaning that cannot be derived from the conjoined meanings of its elements or in its grammatically atypical use of words.