Praise be to Allah.
Belief in the Last Day, and what is connected to it of belief in Paradise and Hell, is one of the six pillars of faith. Whoever does not believe in any of these six pillars is a disbeliever in Allah and will abide forever in Hell; no righteous deeds will benefit him and on the Day of Resurrection no intercessor will be able to help him.
Allah, may He be exalted, says (interpretation of the meaning):
“The Messenger has believed in what was revealed to him from his Lord, and [so have] the believers. All of them have believed in Allah and His angels and His books and His messengers, [saying], ‘We make no distinction between any of His messengers.’ And they say, ‘We hear and we obey. [We seek] Your forgiveness, our Lord, and to You is the [final] destination’”
And Allah, may He be exalted, says describing the believers (interpretation of the meaning):
“… and of the Hereafter they are certain [in faith]”
It was narrated from Abu Hurayrah (may Allah be pleased with him) that one day the Messenger of Allah (blessings and peace of Allah be upon him) appeared before the people and there came to him a man who said: O Messenger of Allah, what is faith? He said: “Faith is to believe in Allah, His angels, His Books, His Messengers, and the meeting with Him, and to believe in the resurrection hereafter.” Narrated by al-Bukhaari (4777) and Muslim (9).
According to the famous hadith of Jibreel, which was narrated by Muslim (8), [Jibreel] said: Tell me about faith (eemaan). He said: “It is to believe in Allah, His angels, His Books, His Messengers, the Last Day, and to believe in al-qadar (the divine will and decree), both good and bad.” He said: You have spoken the truth.
Belief in the Last Day is part of belief in Allah; so whoever does not believe in the Last Day is not a believer in Allah. Allah, may He be exalted, says (interpretation of the meaning):
“… but [true] righteousness is [in] one who believes in Allah , the Last Day, the angels, the Book, and the prophets… ”
“And [also] those who spend of their wealth to be seen by the people and believe not in Allah nor in the Last Day. And he to whom Satan is a companion - then evil is he as a companion”
“Fight those who do not believe in Allah or in the Last Day and who do not consider unlawful what Allah and His Messenger have made unlawful and who do not adopt the religion of truth from those who were given the Scripture - [fight] until they give the jizyah willingly while they are humbled”
And Allah, may He be exalted, says (interpretation of the meaning):
“Those will have lost who deny the meeting with Allah , until when the Hour [of resurrection] comes upon them unexpectedly, they will say, ‘Oh, [how great is] our regret over what we neglected concerning it,’ while they bear their burdens on their backs. Unquestionably, evil is that which they bear”
“… while they were, concerning the Hereafter, disbelievers”
“Who took their religion as distraction and amusement and whom the worldly life deluded." So today We will forget them just as they forgot the meeting of this Day of theirs and for having rejected Our verses”
“Those who disbelieve have claimed that they will never be resurrected. Say, "Yes, by my Lord, you will surely be resurrected; then you will surely be informed of what you did”
“Every soul, for what it has earned, will be retained
Except the companions of the right,
[Who will be] in gardens, questioning each other
About the criminals,
[And asking them], ‘What put you into Saqar?’
They will say, ‘We were not of those who prayed,
Nor did we used to feed the poor.
And we used to enter into vain discourse with those who engaged [in it],
And we used to deny the Day of Recompense
Until there came to us the certainty’”
And there are many similar verses and hadiths.
Muslim (214) narrated from ‘Aa’ishah [that said said:] I said: O Messenger of Allah, during the Jaahiliyyah Ibn Jud‘aan used to uphold the ties of kinship and feed the poor. Will that benefit him at all? He said: “It will not benefit him, because he did not say one day, ‘Lord forgive me my sins on the Day of Judgement.’”
An-Nawawi (may Allah have mercy on him) said:
What this hadith means is that what he used to do of upholding ties of kinship, feeding the poor and other noble traits will not benefit him in the hereafter, because he was a disbeliever. This is what is meant by the words of the Prophet (blessings and peace of Allah be upon him) “because he did not say one day, ‘Lord forgive me my sins on the Day of Judgement’”, i.e., he did not believe in the resurrection, and whoever does not believe in it is a disbeliever, whose good deeds will not benefit him.
Al-Qaadi ‘Iyaad (may Allah have mercy on him) said: There is scholarly consensus that the disbelievers’ good deeds will not benefit them and they will not be rewarded for them with bliss or with reduction of the punishment. End quote.
Shaykh Ibn Baaz (may Allah have mercy on him) said:
Whoever doubts the hereafter and doubts whether Paradise exists or not, or whether Hell exists or not, or whether there will be a resurrection or not, or whether Allah will resurrect the dead, has no faith or certainty. Such a person is a disbeliever until he believes in the resurrection and in Paradise and Hell, and that Allah has prepared Paradise for the righteous and has prepared Hell for the disbelievers. It is essential to believe in that, according to the consensus of the Muslims.
End quote from Majmoo‘ Fataawa Ibn Baaz (28/231).
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
Allah often mentions belief in Him, may He be exalted, alongside belief in the Last Day, and belief in the initial creation alongside belief in the re-creation (resurrection), because whoever does not believe in the Last Day cannot be a believer in Allah.
End quote from Majmoo‘ Fataawa wa Rasaa’il al-‘Uthaymeen (8/472).
And Allah knows best.
See also the answers to questions no. 12295, 12347 and 13994.