Is it possible for one who dies without repenting from a major sin to be definitively saved from the torment of the grave and Hell?

Whoever among the monotheists dies having committed a major sin without repenting from it, then their fate is subject to the will of Allah the Almighty. If He wills, He will forgive them and admit them to Paradise. If He wills, He will punish them and then admit them to Paradise due to their monotheism, for monotheism is the greatest cause of salvation and forgiveness.

The punishment for sins is removed from a servant by ten causes: repentance, seeking forgiveness, effacing good deeds, the supplication of believers for them, righteous deeds performed for the deceased, calamities that atone for sins, and this includes Allah's mercy, pardon, and forgiveness. The punishment for a disobedient believer in the grave may also be lightened.