Is the questioner considered sinful against his father who committed suicide, and how can he atone for his father's sins after he committed suicide and burned the house?

May Allah ease your sorrow over your father's passing, and we advise you to be patient and seek reward from Allah. You did well in honoring your father during his lifetime, and there is no sin upon you if you did not commit any disobedience or ill-treatment towards him.

If your father died in the state you mentioned, that he was secular, did not pray, and mocked those who believed in Allah and His Messenger or prayed, then it is not permissible for you to supplicate for him, seek forgiveness for him, or perform any righteous deeds on his behalf. This is based on the Almighty's saying: "It is not for the Prophet and those who have believed to ask forgiveness for polytheists, even if they were relatives, after it has become clear to them that they are companions of Hellfire."

However, if he repented and returned to the truth, then there is no objection to supplicating for him, seeking forgiveness for him, and honoring him after his death.