Imam has undertaken very useful discussions in his famous book “an exposition on Forty Hadith” 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.