As for that with which God—may His praise be exalted—described the believers (in that which was revealed to His Prophet Muḥammad, Peace and blessings of God be upon him, and that which was revealed to the messengers before him) regarding their certainty concerning the matter of the Hereafter, it is their certainty in that which the polytheists denied: resurrection, raising from the dead, reward, punishment, reckoning, the Scale, and other such things that God has prepared for His creation on the Day of Resurrection. Just as:

291- Muḥammad ibn Ḥumayd narrated to us, saying: Salama narrated to us, from Muḥammad ibn Isḥāq, from Muḥammad ibn Abī Muḥammad, the freed slave of Zayd ibn Thābit, from ʿIkrima, or from Saʿīd ibn Jubayr, from Ibn ʿAbbās: {And of the Hereafter they are certain} (4): meaning, in resurrection, the Day of Resurrection, Paradise, Hellfire, the reckoning, and the Scale; that is, not those who claim that they believe in what was before you while disbelieving in what came to you from your Lord. (1)

This interpretation from Ibn ʿAbbās has explicitly shown that the Surah from its beginning—even though the verses at its beginning describe the believers—is an exposure from God, the Mighty and Majestic, criticizing the disbelievers among the People of the Book who claimed that they were believers in what was brought by the messengers of God—may God's blessings be upon them and upon him—who came before Muḥammad, while they disbelieved in Muḥammad (peace and blessings of God be upon him) and rejected the Revelation he brought, yet claiming despite their rejection that they were rightly guided, and that none would enter Paradise except one who was a Jew or a Christian. So God—may His praise be exalted—belied that assertion of theirs with His statement: {Alif, Lām, Mīm. That is the Book about which there is no doubt, a guidance for the God-fearing, who believe in the unseen, establish prayer, and spend out of what We have provided them, and who believe in what has been revealed to you and what was revealed before you, and of the Hereafter they are certain} (1-4). And He—may His praise be exalted—informed His servants that this Book is a guidance specifically for the people of faith in Muḥammad (Peace and blessings of God be upon him) and in what he brought, who believe in the clear proofs and guidance revealed to him and to His messengers before him—to the exclusion of whoever disbelieved in Muḥammad (Peace and blessings of God be upon him) and in what he brought, while claiming to believe in the messengers before Muḥammad (peace and blessings of God be upon him)