What is the reason for God's rejection of the Prophet's plea for forgiveness for his mother, and for her being considered a disbeliever despite her death before the advent of prophethood? Does this mean she was not a follower of Christianity?

The father of the Prophet, peace and blessings be upon him, died before his birth. Muslim narrated from Anas that a man asked the Prophet about his father, and he said: "In the Fire." When the man turned to leave, the Prophet called him and said: "Indeed, my father and your father are in the Fire." This indicates that whoever dies upon disbelief will be in the Fire, even if they died during a period when revelation was intermittent, because the call of the prophets, such as Abraham and others, had reached them, and dying upon polytheism warrants punishment. As Ibn al-Qayyim mentioned, the polytheist deserves punishment for opposing the call of the Messengers, for Allah has the conclusive argument against polytheists at all times.