[2:4] and who believe in what has been revealed to you; and what was revealed before you; and of the Hereafter, they are certain.
Their belief in the Unseen necessarily requires their belief in the QurÌān and in what God has revealed in the books before the QurÌān, but He repeats the mention of belief here for the purposes of specification and corroboration. The affirmation (taṣdīq) of the intermediary (ṣ) in some of what he has communicated makes the affirmation of him obligatory in all that he has communicated, since the evidence of his sincerity bears witness generally without any restriction. They have certainty in the Hereafter because they have witnessed the Unseen: When the Messenger of God (ṣ) asked Ḥāritha, 'How is your state?' He said, 'I have become one who submits to God in truth and it is as if I were with the people of the Garden exchanging visits, and as if I were with the people of the Fire clamoring to one another, and as if I were with the throne of my Lord in plain sight. The Messenger of God (ṣ) said, 'You have it right, so keep to it'.
ʿĀmir b. ʿAbd al-Qays said, 'If the veil were to be lifted, it would not increase my certainty'. The real meaning of certainty is to rid yourself of indecisive guessing and to stay far away from loose conjecturing.