Question: Does īmān [belief, faith] increase or decrease?
Hadrat Imām-i Rabbānī declares:
Since īmān is a confirmation and belief in the heart, there is no increase or decrease in it. Belief that increases and decreases cannot be īmān. It is called zan [supposition]. By performing the worships, the actions which Allahu taālā loves, īmān gets polished, becomes luminous, and shines. By committing the harāms, it becomes cloudy. Then an increase or decrease occurs in the luster of the īmān on account of behavior. It does not happen in itself. Some (savants) said that the īmān which is bright is very much greater than that which is not bright. They almost did not consider some beliefs that are not bright as īmān. And about some lustrous ones they said that they were īmān but that they were small. This case with īmān is like the case of two mirrors standing opposite each other with different brightnesses. It is similar to saying that the one which is more lustrous and reflects the things opposite itself more brightly is greater than the one that reflects less brightly. And another person, for example, says that the two mirrors are equal. They are different only in brightness, in reflecting the things opposite themselves; that is, their properties, qualities are different. Of the two persons, the former has looked at the appearance and has not penetrated into the essence. The hadīth-i sharīf The īmān of Abū Bakr-i Siddīq (radiy-Allahu anh) is heavier than the total sum of the īmāns of all my Ummat is in regard to the nūr, the luster of īmān. (Maktūbāt, 266th Letter)