This is the second basis for the movement that Imam Hussein (AS) launched against Yazid as mentioned in the third Imam’s (AS) letter to his brother Muhammad ibn al-Hanafiyya.
According to some Hadiths, enjoining good and forbidding evil is like a sea before which other good deeds are but a drop of water.
The Holy Quran in many verses refers to enjoining good and forbidding evil as a duty of all believers.
For instance, God say sin Verse 71 of Surah At-Tawbah: “And (as for) the believing men and the believing women, they are guardians of each other; they enjoin good and forbid evil and keep up prayer and pay the poor-rate, and obey Allah and His Messenger; (as for) these, Allah will show mercy to them; surely Allah is Mighty, Wise”
According to this verse, ordering believers to enjoin good and forbid evil is because there is friendship and cooperation among believers in society. The believers are each other’s Wali (friend) and this means that they should help each other in various matters.
They should urge one another to do good and prevent one another from doing evil. So believers feel responsible for one another and will not be upset by what their brother in faith tells them to do or not to do.
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However, if the ruler of the society is a corrupt and oppressive person like Yazid ibn Muawiyyah, he would prevent the formation of a society of faith and, as a result, the formation of friendship and brotherhood among believers. He would try to bring the society under his control by fomenting division and discord among different groups and persecuting true believers like Imam Hussein (AS). At this point, a person like Imam Hussein (AS) considers it his duty to rise up against a corrupt ruler like Yazid and pave the way for the realization of a society of faith, brotherhood, and friendship by removing him from power. Hence, the greatest form of enjoining good and forbidding evil is to rise up against an oppressive ruler.