preach to the converted

Translations
проповедовать обращённым
Talking about the benefits of exercise to athletes is like preaching to the converted.
Говорить об пользе упражнений спортсменам — это как проповедовать обращённым.
Definitions
To speak to an audience that already shares the same beliefs or opinions, often resulting in a lack of new insights or persuasion.
Words
preach

verb
проповедовать
to

preposition
к
the

article or possessive pronoun
этим
converted

noun
убеждённым