Îmân does not increase or decrease

Question: Does îmân [belief, faith] increase or decrease?
Hadrat Imâm-i Rabbânî declares:
Since îmân is a confirmation and belief in the heart, there is no increase or decrease in it. Belief that increases and decreases cannot be îmân. It is called zan [supposition]. By performing the worships, the actions which Allahu ta’âlâ loves, îmân gets polished, becomes luminous, and shines. By committing the harâms, it becomes cloudy. Then an increase or decrease occurs in the luster of the îmân on account of behavior. It does not happen in itself. Some (savants) said that the îmân 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 îmân. And about some lustrous ones they said that they were îmân but that they were small. This case with îmân 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 hadîth-i sharîf “The îmân of Abû Bakr-i Siddîq (radiy-Allahu ’anh) is heavier than the total sum of the îmâns of all my Ummat” is in regard to the nûr, the luster of îmân. (Maktûbât, 266th Letter)