What is the ruling on dealing with a father who curses and insults religion and God—God forbid—when angry, and how can he be advised without angering him or being undutiful, and is it permissible to pray behind him as an Imam?

Cursing the religion is a major disbelief (kufr akbar) and apostasy from Islam. It is not permissible to pray behind someone who commits this act, if they are conscious, unless they repent. One must denounce whoever curses the religion and command them to repent, with its conditions: ceasing the sin, regretting it, and resolving not to return to it. Scholars have unanimously agreed that cursing or mocking the religion is apostasy that expels one from the fold of Islam. The perpetrator of this act is asked to repent; if he repents, his repentance is accepted by Allah, but if he does not repent, he is to be executed. The father should be advised with wisdom and gentleness, explaining the seriousness of his action, encouraging him to repent and associate with the righteous, and supplicating for his guidance.