Belief in the Last Day is one of the six pillars of faith without which a person’s faith cannot be completed and accepted. This is proven by the Qur’an and Sunnah, and by scholarly consensus.

2.1. Evidence from the Qur’an
Allah, may He be Glorified and Exalted, says (interpretation of the meaning), "And whoever disbelieves in Allah, His angels, His books, His messengers, and the Last Day has certainly gone far astray." (al-Nisa’ 4:136) Ibn Jarir said, “No one’s faith can be valid unless he believes in what Allah has enjoined him to believe. Disbelieving in any part of that is disbelieving in all of it.” Al-Shawkani said, “And whoever disbelieves in Allah, His angels, His books, His messengers, and the Last Day that is, whoever disbelieves in any part of that is misguided and has gone far astray.” And Allah, may He be Glorified and Exalted, says (interpretation of the meaning), "Those who do not expect to meet Us, and are pleased and satisfied with the life of this world, and are heedless to Our signs, their abode will be the fire, because of their misdeeds." (Yunus 10:7-8)

2.2. Evidence from the Sunnah
It was narrated from ‘Umar ibn al-Khattab (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said, defining faith (iman), “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.” (Muslim)

2.3. Scholarly consensus
Abu’l-Hasan al-Ash‘ari said, “The scholars are unanimously agreed that the torment of the grave is true, and that people will be subject to trial in their graves after they are brought back to life therein; they will be questioned and Allah will make steadfast whomever He wants to make steadfast. They will not taste the pain of death a second time, as Allah, may He be Glorified and Exalted, says (interpretation of the meaning), "They will not taste death therein, except the first death." (al-Dukhan 44:56) The Trumpet will be blown before the Day of Resurrection, and all those who are in the heavens and on earth will swoon, except whomever Allah wills. Then it will be blown again, whereupon they will be standing, looking around. Allah, may He be Glorified and Exalted, will recreate them as He created them the first time, barefoot, naked, and uncircumcised. The same bodies that obeyed Him or disobeyed Him will be resurrected on the Day of Resurrection, and the same skins, tongues, hands, and feet that they had in this world will testify against them on the Day of Resurrection. Allah, may He be Glorified and Exalted, will set up the balances to weigh people’s deeds. The one whose good deeds weigh heavily will succeed, and the one whose good deeds weigh lightly will lose and be doomed. If the pan containing bad deeds is heavy, it will sink down to Hell, and if the pan containing good deeds is heavy, it will drop all the way and land in Paradise.

On the Day of Resurrection people will be given the records containing their deeds. Those who are given their records in their right hands will have an easy reckoning, whereas those who are given their records in their left hands will be thrown into Hell. Ibn Hazm said, “The scholars are unanimously agreed that the resurrection is true and that all people will be resurrected.” Ibn al-Qayyim said, “The three principles on which all religions are agreed, and which all the Messengers taught, are: belief in Allah and the Last Day, and doing righteous deeds. Allah, may He be Glorified and Exalted, says (interpretation of the meaning), "Those who believe in the Prophet, and those who are Jews, and the Christians and the Sabians – whoever among them believed in Allah and the Last Day, and did righteous deeds, will have their reward with their Lord; they will have no fear, nor will they grieve." (al-Baqarah 2:62)