One type of disbelief (kufr) is disbelief because of doubt

Whoever has doubts about Allah, the angels, the messengers, the resurrection, Paradise, Hell, or anything that has reached him of what Allah said (in the Quran) or that was conveyed by His Messenger (blessings and peace of Allah be upon him) is a disbeliever.

Allah, may He be exalted, says (interpretation of the meaning):

“And he entered his garden while he was unjust to himself. He said, ‘I do not think that this will perish – ever.

And I do not think the Hour will occur. And even if I should be brought back to my Lord, I will surely find better than this as a return.’

His companion said to him while he was conversing with him, "Have you disbelieved in He who created you from dust and then from a sperm-drop and then proportioned you [as] a man’” [al-Kahf 18:35-37].

Muslim (27) narrated from Abu Hurayrah that the Prophet (blessings and peace of Allah be upon him) said: “I bear witness that there is no god but Allah and that I am the Messenger of Allah. No one will meet Allah with this (twin declaration of faith), not doubting it, but he will enter Paradise.”

Al-Bukhaari (86) and Muslim (905) narrated that the Prophet (blessings and peace of Allah be upon him) said: “It was revealed to me that you will be tested in your graves; it will be said: What do you know about this man? The believer or the one who has certain faith will say: He is Muhammad, the Messenger of Allah; he came to us with clear signs and guidance, and we responded and followed (him); he is Muhammad – three times. Then it will be said to him: Sleep the sleep of the righteous, for we knew that you believed with certain faith in him. As for the hypocrite or the doubter, he will say: I do not know, I heard the people saying something so I said it.”

Shaykh ‘Abd al-‘Azeez ar-Raajihi (may Allah preserve him) said:

The doubter is regarded as a disbeliever if he has doubts about Allah, or about the angels or the books or the messengers or Paradise or Hell, and he says: I do not know whether there is a paradise or not, or whether there is a hell or not. He becomes a disbeliever if he doubts in this manner. End quote.

On this basis we may understand the report narrated by Ibn Maajah (4268) from Abu Hurayrah from the Prophet (blessings and peace of Allah be upon him), who said: “The dead person ends up in his grave, then the righteous man is made to sit up in his grave with no fear or panic … And the evil man is made to sit up in his grave with fear and panic. It is said to him: ‘In what [condition] were you?’ He says: ‘I do not know.’ It is said to him: ‘Who is this man?’ He says: ‘I heard the people saying something and I said it too.’ Then a window to Paradise is opened to him, and he looks at its beauty and what it contains. It is said to him: ‘Look at what Allah has diverted from you.’ Then a window to Hell is opened for him, and he sees it, parts of it destroying others, and it is said to him: ‘This is your place. You were doubtful; in this state you died and in this state you will be resurrected, if Allah, may He be exalted, wills.’”

Classed as saheeh by al-Albaani in Saheeh Ibn Maajah.

As-Sindi (may Allah have mercy on him) said:

This hadith indicates that whoever has certain faith in this world will usually die in that state, and the same applies to one who has doubt.

End quote from Haashiyat as-Sindi ‘ala Ibn Maajah (2/568)

So whoever doubts one of the fundamental beliefs and dies in a state of doubt is a disbeliever who will abide in Hell forever, for Allah does not accept anything from His slave except certain faith.

And Allah knows best.