What is the ruling on insulting a disbelieving father who curses the Divine Being, religion, and Muslims? Is that considered a major sin, and how does one perform sincere repentance from it?

Cursing the religion or Allah is disbelief that expels one from the faith. The father who falls into this disbelief must be advised, and the gravity of his action must be explained to him, while maintaining the prohibition of reviling, cursing, or mistreating him, for parents have a great right.

If you have previously cursed or reviled your father, repentance involves abandoning the act, regretting it, and resolving never to return to it, as well as seeking your father's forgiveness and pardon.

If the cursing occurred without his knowledge, there is no need to seek his forgiveness. It is better to avoid the topic and supplicate for his guidance.