"The slave is not afflicted with a punishment greater in severity then the hardening of the heart and being distant from Allah...

ibn al Qayyim al Jawziyyah’s al Fawaaid (p 111-112) "The slave is not afflicted with a punishment greater in severity then the hardening of the heart and being distant from Allah. For the Fire was created to melt the hardened heart. The most distant heart from Allah is the heart which is hardened. If the heart becomes hardened, the eye becomes dry. If four matters are exceeded in quantity, beyond what is necessary, the heart shall become hardened: Food, sleep, speech and sexual intercourse. A body afflicted by disease does not derive nourishment from food or water, similarly a heart diseased by desire does not benefit from admonishment and exhortation. Whosoever desires to purify his heart, then let him prefer Allah to his desires. The heart which is clinging to its desires is veiled from Allah, commensurate to the degree that it is attached to them, the hearts are the vessels of Allah upon his earth, hence the most beloved of them to him, are the ones most compassionate, pure and resistant to deviation. They (the transgressors) preoccupied their hearts [in the pursuance] of the Dunya, would that they preoccupied them with Allah and the hereafter, then surely they would have reflected upon the intended meaning of his poignant words and verses. Their hearts would have returned to their masters with wisdom, marvelously curious and [in possession] of the rarest of precious gems. If the heart is nourished with remembrance, its thirst quenched with contemplation and cleansed from corruption, it shall witness remarkable and wondrous matters, inspiring wisdom. Not every individual who is endowed with knowledge and wisdom, and assumes its character is from amongst its people. Rather the people of knowledge and wisdom are those who infused life into their hearts by slaying their desires. As for the one who slays his heart and vitalized his desires, then knowledge and wisdom is naked upon his tongue. The destruction of the heart occurs by security [in this Dunya] and negligence, its fortification occurs by fear and remembrance. If the heart renounces the pleasures of the Dunya, it settles upon the [pursuance of] pleasures of the Hereafter, and amongst those who call towards it. Should the heart become content with the pleasures of the Dunya, those pleasures [of the hereafter] cease [to continue]. Yearning for Allah and his meeting is like the gentle breeze blowing upon the heart, extinguishing the blaze of the Dunya. Whosoever caused his heart to settle with his Lord shall be in a state, clam and tranquil, and whosoever sent it amongst the people shall be disturbed and excessively perturbed. For the love of Allah shall not enter a heart, which contains the love of this world, except as a camel, which passes through the eye of a needle. Hence, the most beloved servant before Allah is the one whom He places in His servitude, whom He selects for His love, whom He causes to purify his worship for Him, dedicates his objectives for Him, his tongue for His remembrance, and his limbs for His service. The heart becomes sick, as the body becomes sick, and its remedy is al-Tawbah and protection [from transgression]. It becomes rusty as a mirror becomes rusty, and its clarity is obtained by remembrance. It becomes naked as the body becomes naked, and its beautification is at-Taqwa. It becomes hungry and thirsty as the body becomes hungry and thirst, and its food and drink are knowledge, love, dependence, repentance and servitude.”