What is the ruling on those who do not believe in matters of the creed, such as the torment in the grave, and reject them alleging that such issues are based on Aahaad Hadeeths (Hadeeths whose chains of transmission consist of few narrators i.e., not more than two narrators in each stage of the chain), and claiming that such type of Hadeeths do not suggest decisive knowledge?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.

As long as an Aahaad Hadeeth is authenticated, it takes the status of the Mutwaatir Hadeeths (consecutively transmitted narrations) concerning the obligation to believe in it and apply its implications.

Ibn Abi Al-'Izz  (Abil’izz) stated that if a report, even if it is not narrated through multiple narrators, but is unanimously accepted as authentic and acted upon by the Ummah (Muslim nation), then this implies its authenticity as if it were a Mutawaatir Hadeeth. This is something that the righteous predecessors unanimously agreed upon.

In addition, Al-Haafith Ibn Hajar mentioned that the Companions and the Taabi‘oon (followers) used to act upon Aahaad Hadeeths as long as they are authentic without any of them opposing this. This indicates their agreement in accepting such Hadeeths and applying them.

Therefore, there is no difference in this regard between matters of the creed and rulings of Fiqh (Islamic jurisprudence), as long as the Hadeeth is authentic.

With regard to the bliss and torment of the grave, we want to draw your attention to the fact that it is obligatory to believe in it, as this is proved by the Mutwaatir Hadeeths, as stated by Ibn Rajab . It was narrated on the authority of ‘Aa’ishah, the Mother of the Believers that the Prophet stated that the torment of the grave is true. [Al-Bukhari and Muslim]

It  was also narrated on the authority of Anas that the Prophet mentioned that when the deceased person is laid in his grave and people leave him, two angels will come to him and ask: "What did you say about this man, Muhammad, sallallaahu ‘alayhi wa sallam?” If he is a Muslim, he will answer that he testifies that he is the Messenger of Allaah, sallallaahu 'alayhi wa  sallam.  Then the two angels will give him glad tidings that he will enter Paradise. However, if he is a disbeliever, he will not be able to answer the question.  Then he will be severely hit with an iron hammer. [Al-Bukhari]

In conclusion, a person who denies the torment or bliss of the grave because he does not know that it has been authenticated in the Sharee'ah (Islamic law) will be excused for his ignorance.  However, if he denies this, despite his knowledge of it, and insists on the denial, then he is a disbeliever for rejecting an unequivocal precept of the religion.

Allaah Knows best.