Imam Khomeini explains the matter through his famous work "an exposition on Forty Hadith" as following:
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.