What is the ruling on those who do not accept sound Ahadith in matters of creed, such as not believing in the torment of the grave or not denying it?

If an Ahad hadith (a solitary narration) is authentically attributed, it conveys certain knowledge and necessitates belief in it and acting upon its implications, whether it pertains to creed or other matters, just like a Mutawatir hadith (a consecutively narrated hadith). Narrations regarding the punishment and reward in the grave have reached us from the Prophet, peace and blessings be upon him, through Tawatur. Belief in it is obligatory. Whoever denies it out of ignorance is excused, but whoever denies it knowingly is considered a disbeliever.