Why did Abraham (peace be upon him) not get angry with his father?

Abraham, peace be upon him, is a role model in calling to monotheism. He debated with his father and his people with gentleness and strong arguments, despite their animosity towards him. This is what callers [to Islam] must learn in their discourse. Furthermore, his debate with his father was an example of etiquette and reverence, while his father responded with threats. Abraham, in turn, responded with supplication and seeking forgiveness for him. It is not permissible for a believer to seek forgiveness for a disbeliever, except in the case of Abraham with his father, due to a promise he had made. However, when it became clear to him that he was an enemy of Allah, he disassociated himself from him.