Belief in the unseen (Al-Ghayb) is the sine qua non, a fundamental condition of faith in Islam. Having faith in a vast, unseen world which cannot be comprehended by our sense of perception is a requirement for having faith in Allah. "This Book, there is no doubt in it, is a guide to those who guard (against evil). Those who believe in the unseen and keep up prayer and spend out of what We have given them." (Holy Qur'an, 2:2-3) These verses lay emphasis upon the fact that belief in the unseen is a part of faith.
A person is a believer if he possesses faith. Faith is born into the heart and is a genuine, spiritual state. "The dwellers of the desert say: We believe. Say: You do not believe but say, We submit; and faith has not yet entered into your hearts; and if you obey Allah and His Messenger, He will not diminish aught of your deeds; surely Allah is Forgiving, Merciful." (49:14) This verse of Sura Hujaraat distinguishes between the believer and the one who has accepted Islam. By proclaiming the testimonies of faith, one enters Islam; however, it is when true faith springs up in the heart that one becomes a believer. One fundamental element of faith is belief in the unseen. This unseen does not relate to those material objects which exist but at some point are not in our knowledge, and about which knowledge can be acquired with our sense of perception; rather, this unseen refers to all those realities and facts that cannot be perceived by our senses. These realities are present around us but our sense of perception does not have the ability to grasp them.
The scope of faith in the unseen includes, but is not limited to, faith in God, the Revelation, the Divine books, angels, the Day of Resurrection, Imam Muhammad al-Mahdi (may Allah hasten his reappearance), the help of Allah, the promise of Allah, the might and mercy of Allah, and life after death, among other things. It includes all those facts which cannot be perceived by material senses.
It is reported that once someone asked the Commander of the Believers, Imam Ali (peace be upon him): "Have you seen your Lord?" The Imam answered: "I will never worship a Lord whom I cannot see." The man then asked: "How did you see him? Explain it to us." He replied, "Woe upon you! No one has ever seen Him with the physical eye, but hearts filled with the truth of faith have contemplated Him."
How does this truth of faith emerge in our hearts? And why do so many people deny the realities and facts related to the unseen? The answers to these questions lie in the issue of guidance and deviation. There are two types of guidance and deviation. The first guidance comes from the unpolluted and clean heart of a man. If he has not corrupted his soul and his thinking has not been affected with darkness of ignorance, he advances towards truth, and this advancement towards truth makes him eligible for the second level of guidance from Allah. The same is true for unbelievers and their disbelief. First they corrupted themselves and deviated from the right path, and at the second level they were further pushed into error.
With respect to the beginning verses of Sura Baqara, Allama Tabatabai discusses in Al-Mizan: "In short, the pious ones are surrounded by two guidance, as the disbeliever and hypocrites fall between two errors. The second guidance is by the Qur'an; therefore, the first one must have been before the Qur'an. They must have been guided by a healthy and unimpaired psychology. If a man's nature is faultless and flawless, it cannot fail to see that it is dependent on some thing above it. Also, it realizes that every other thing, which it may perceive, imagine or understand, depends likewise on a thing outside the chain of dependent and needy things. Thus, it comes to believe that there must be a Being, unseen and un-perceptible through the senses, who is the beginning and end of every other thing. It also sees that the said Essential Being does not neglect even the smallest detail when it comes to creative perfection of His creatures. This makes him realize that the said Creator cannot leave the man to wander aimlessly hither and thither in his life; that He must have provided for him a guidance to lead him aright in his actions and morals. By this healthy reasoning, the man acquires the belief in One God, in the institution of prophethood and in the Day of Resurrection. In this way, his faith in the fundamentals of religion becomes complete. That faith leads him to show his servitude before his Lord, and to use all that is in his power - wealth, prestige, knowledge, power, and any other excellence - to keep this faith alive and to convey it to others. Thus we come to the prayer and benevolent spending. The five virtues enumerated in these verses are such that a healthy nature unfailingly leads the man to them. Once a man reaches this stage, Allah bestows on him His another grace, that is, the guidance by the Qur'an."
Therefore we can conclude that belief in the unseen can be attained only with uncorrupted soul and unbiased, clear, logical thinking.