Is one who witnesses a brawl involving the cursing of religion, whether at home or in the street, sinful, and does this fall under the saying of Allah the Almighty: "Indeed, then you would be like them"?

You must advise those who commit evil and forbid them from it. If they do not respond, then there is no sin upon you. However, merely witnessing without enjoining good and forbidding evil is not permissible. Allah the Almighty guided us in Surat Al-An'am to remind and advise the one who commits evil, so that perhaps they may fear Allah. He said: "And when you see those who engage in [offensive] discourse concerning Our verses, then turn away from them until they enter into another conversation. And if Satan should cause you to forget, then do not remain after the remembrance with the wrongdoing people." Then He said: "And those who fear Allah are not accountable for them in anything, but [theirs is] only a reminder that perhaps they will fear Allah." Al-Sa'di explained this by saying that whoever sits with them and remains silent regarding the evil is sinful, but whoever enjoins good and forbids them from evil, there is no blame upon him.