It is an everyday experience that when you do not know a thing, you look for somebody who knows it and you repose faith in his word and follow him.
If you get ill and you cannot treat and cure yourself, you look for the doctor and accept and follow his instructions without question.
Because he is properly qualified to give medical advice, possesses experience, and has treated and cured a number of patients.
You, therefore, stick to his advice, do whatever he asks you to do, and avoid whatever he forbids.
Similarly, in matters of law you believe in whatever your legal advisers say and act accordingly. In educational matters you have faith in your teacher or instructor and you accept his instructions as true.
When you want to go to some place and do not happen to know the way to it, you ask somebody who knows it and follow the way he points out.
In short, the reasonable course that you adopt in the whole of your life about matters which you do not or cannot know is that you approach one who knows them, accept his advice, and act accordingly.
As your own knowledge of that matter or problem is inadequate, you carefully search for one who knows that and then silently accept his word.
You take every pain to select the proper person. But after selecting the right man, you accept his advice unquestioningly.
This kind of belief is called “belief in the unknown.” For here you have relied upon one who knows, in matters you do not know.
This exactly is iman bil ghaib. This signifies that you achieve the knowledge of what was not known to you from one who knows.
You do not know God and His real attributes. You are not aware that His angels are directing the machinery of the whole universe according to His orders, and that they surround you on all sides.
You have not the proper knowledge of the way of life through which you can seek the pleasure of your Creator. And you are in the dark about the life that is to come.
The knowledge of all the mentioned matters is given to you by the prophets, who had direct contact with the Divine Being and had been endowed with the correct knowledge.
And they are the persons whose sincerity, integrity, trustworthiness, godliness, and absolute purity of their lives stand as irrevocable witness to the truth of their claim to knowledge.
And above all, the very wisdom and force of their message makes you admit that they speak the truth and their preaching deserves to be believed and followed.This conviction of yours is iman bil ghaib.
Such truth-discerning and truth-acknowledging attitude is essential for obeying God and for acting in accordance with His pleasure, for you have no other medium than God’s Messenger for the achievement of true knowledge, and without accurate and true knowledge you cannot proceed rightly on the path of Islam.