Is it true that if a disbeliever's heart were transplanted into a believer, the latter would not change, based on the verse "but it is the hearts which are within the breasts that become blind" and the Hadith "Indeed, there is a piece of flesh in the body"?

The heart was named so either because of its constant change (taqallub) or because it is the purest part of the body. It is the body's ruler; its soundness makes the body sound, and its corruption corrupts it. Through it, the servant understands and directs himself.

Any thought or idea that crosses one's mind is due to a change in the heart, not the body. The heart of a disbeliever (kafir) and a hypocrite (munafiq) darkens according to what feeds it, and if the corrupting substances change, it becomes sound. A transplanted heart may transfer some traits of its original owner; some doctors have observed changes in patients' behaviors after heart transplantation.

The relationship between the heart and the body has a material, organic aspect and an unseen, spiritual aspect. Happiness and misery are predetermined for the body's owner and are linked to his earning and deeds. The heart is a repository and a center of initiation. The new heart performs its spiritual functions by precise divine decree and becomes sound in its new owner according to the substance that feeds it. It is a borrowed heart that will return to its original owner in the Hereafter. A disbeliever cannot become a Muslim by receiving a Muslim's heart, nor vice versa.