Imam Khomeini explans the matter through his famous book “Combat with the self” as following:
Sometimes you see that one has knowledge of these realities but has no faith.
Undertakers are not afraid of the dead, for they have certainty that the dead cannot harm one, even when he was alive and had a spirit in his body he was not harmful, so what harm can he be now as an empty frame? However, those who are afraid of the dead are afraid because they do not have faith in this truth. They merely have knowledge.
They know about God and the Day of Atonement, but they lack certainty.
The heart is unaware of that which the intellect has understood.
They know the proofs for the existence of God and the reality of the Resurrection, but these very same intellectual proofs may be veils covering the heart which do not permit the light of faith to shine in, Imam added.
Until God, the Exalted, frees them from the darkness and the obscurities and leads them to enter the worlds of light and radiances:
“Allah is the Guardian [wali] of those who believe; He brings them out of the darkness into the light” (Q 2:257).
He whose Guardian [wali] is God, the Blessed and Exalted, and who is taken by Him out from the darkness never commits another sin, never backbites, never slanders others, and he is never vengeful or envious of his brothers in faith, Imam furhter explained.
His own heart is filled with a feeling of luminosity and he no longer holds the world or what it contains in high esteem.
As Imam ‘Ali (as) said, “If all the world and what it contains were offered to me to cruelly and unjustly take the skin of a grain of barley from the mouth of an ant, I would never accept it.”
But some of you trample over everything, and you backbite the great [scholars] of Islam, Imam added.
If others speak ill of the grocers and perfume sellers on the street and backbite them, for your part some of you relate unfair things, insult and are impudent toward the scholars of Islam, because you are not firmly grounded in faith and you do not believe in [divine] retribution for your own deeds.