How does one denounce evil in one’s heart?.
It means hating the evil and not sitting with those who do it, because sitting with them without denouncing it is like what the Children of Israel did, whom Allaah cursed in the verses (interpretation of the meaning):
“Those among the Children of Israel who disbelieved were cursed by the tongue of Dawood (David) and ‘Eesa (Jesus), son of Maryam (Mary). That was because they disobeyed (Allaah and the Messengers) and were ever transgressing beyond bounds.
79. They used not to forbid one another from Al-Munkar (wrong, evildoing, sins, polytheism, disbelief) which they committed. Vile indeed was what they used to do”
Shaykh ‘Abd-Allaah ibn Baaz.