Imam undertook a series of serious discussions regarding this topic in his famous book of “an exposition on Forty Hadith” and siad in this regard said as following:
Know that faith or belief is something different from the knowledge of God, the Unity of His Being, and His other Attributes--like sifat al-kamaliyyah (the Attributes of Perfection), the Positive Attributes, sifat al-jalaliyyah (the Attributes of Glory), and the Negative Attributes, etc. and the knowledge of angels, holy scriptures, and the Last Day.
Anyone who possesses the knowledge of all these things is not necessarily a believer.
The Devil possesses the knowledge of all these things more than I and you or any other individual, but still he is an unbeliever.
The faith is a yearning of the heart-an inner experience, which if not genuine, does not become faith.
Anyone who has gained knowledge of religion through rational demonstrations and arguments, ought to submit to them whole-heartedly with the totality of his being and obey the call of his heart dutifully i.e. with complete surrender to God, with humility and fear-and accept all the responsibilities expected of him sincerely without any question.
Only then he can become a perfect mumin. Culmination of the faith is tranquility and peace of mind.
When the light of faith is stabilized, it brings with it tranquility of the heart, and this is something which does not issue from knowledge.
It is possible that reason may accept something, which is according to its dictates and logic, but the heart may not be ready to agree with it, thus making knowledge useless.
For instance, you know through your reason that a dead person cannot do any harm and that all the dead in the world do not possess any power of action, even as much power as is possessed by a fly, and that all the physical and spiritual faculties depart as soon as one is dead; but since your heart has not accepted it and has not approved of the judgment of the mind, you cannot spend a dark night with a dead body.
But if your heart yields to your mind and approves of its judgment, this job will no more be difficult for you.
After some effort the heart resigns to the dictates of reason, then no dread of the dead remains in the heart.
Therefore, it is clear that the submission, which is the gratification and comfort of the heart, is quite a different thing from knowledge, which is the gratification of the mind.
It is possible that one can logically prove the existence of the Great Intelligent Designer, the Unity of His Divine Being, the Day of Judgment and other true beliefs, but such a belief cannot be considered as faith (iman), and the person cannot be regarded as a mu’min, but he may be counted among the non-believers, hypocrites and polytheists.
Today my eyes and yours are closed, and we do not possess any divine vision. Our terrestrial eyes are unable to perceive, but when the hidden is revealed and the kingdom of heaven is manifested, the physical world vanishes and reality is disclosed; you will realize that you were not a believer in God, and your rational judgment was not congruous with your faith. Unless the words, —there is no god except God— are inscribed on the tablet of the heart with the pen of reason, man is not a true believer in the Unity of God.
When this sacred and holy dictum is impressed on the heart, the domain of the heart turns by itself into the kingdom of God Almighty.
Then only man does not see anyone else as efficient in the realm of truth; he does not expect any rank, any distinction or riches from anybody else; he does not seek for honor and fame with the help of others; and his heart does not become hypocritical and profane.
Therefore, if you see that riya’ is stealing into your heart, you should realize that your heart has not really surrendered itself to reason, and faith has not illuminated your heart yet; for you consider others to be your God and see them as efficient agents in the world, and you do not trust Him, the only God; it means that you have joined the company of hypocrites, polytheists and idolaters.