That is Allah's promise. Allah does not break His promise. But most people do not know it. ( Surat ar-Rum, 30:6)
Allah created humanity and sent Messengers to enjoin good and forbid evil. He also sent Books to guide humanity to the straight path. The Qur'an, the last of these just Books, guides us from the darkness to the light, to the right path.
In the Qur'an, Allah makes promises to both believers and unbelievers. Unless He wills otherwise, unbelievers are promised a stressful life in this world and an eternal punishment in Hell. But because they have no faith in this promise, they assume that they will never be punished. Allah addresses these deluded people in the following terms:
They ask you to hasten the punishment. Allah will not break His promise… ( Surat al-Hajj, 22:47)
Allah has promised those of you who believe and do right actions that He will make them successors in the land, just as He made those before them successors; will firmly establish for them their religion, with which He is pleased; and give them, in place of their fear, security. “They worship Me, not associating anything with Me.” Any who do not believe after that, such people are deviators. ( Surat an-Nur, 24:55)
But as for those who believe and do right actions, We will admit them into gardens with rivers flowing under them, remaining in them timelessly, for ever and ever. Allah's promise is true. Whose speech could be truer than Allah's? ( Surat an-Nisa', 4:122)
The Words of your Lord are perfect in truthfulness and justice. No one can change His Words. He is the All-Hearing, the All-Knowing. ( Surat al-An‘am, 6:115)
Say: “Allah speaks the truth, so follow the religion of Ibrahim, a man of pure natural belief. He was not one of the idolaters.” (Surah Al ‘Imran, 3:95)