Is the failure to warn a person who commits one of the nullifiers of Islam, whether known or unknown, considered an approval of disbelief and a departure from the fold of Islam, similar to one who does not inform a disbeliever who asked him about Islam? And does the ruling differ between knowing the person and not knowing him?

A Muslim must denounce evil to the best of their ability, as it is one of the greatest collective obligations in Islam, based on the words of Allah Almighty: "And let there be [arising] from you a nation inviting to [all that is] good, enjoining what is right and forbidding what is wrong, and those will be the successful" (Quran 3:104), and the words of the Prophet, peace be upon him: "Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart—and that is the weakest of faith."

The questioner's dislike for what happened and their feeling that it constitutes an nullifier of Islam is evidence of their denunciation of it with their heart, and their disapproval and non-acceptance. Therefore, their action does not lead them out of the fold of Islam, based on the words of the Prophet, peace be upon him: "If a sin is committed on earth, he who witnesses it and dislikes it is like one who was absent from it, and he who was absent from it and approved of it is like one who witnessed it," and his words, peace be upon him: "Indeed, rulers will be appointed over you, and you will recognize some of their actions and disapprove of others. So, whoever dislikes it is absolved, and whoever denounces it is safe, but whoever is pleased and follows them [is not]."

The similarity mentioned in the words of Allah Almighty: "And He has already sent down to you in the Book that when you hear the verses of Allah being denied and ridiculed, then do not sit with them until they enter into another conversation. Indeed, you would then be like them" (Quran 4:140) does not absolutely take a servant out of the fold of Islam unless there is satisfaction with their words and acceptance of them. The criterion is contentment with disbelief for complete similarity to be achieved.