First, one must understand the necessity of having courtesy with God and the importance of fulfilling its requirements. Second one must be aware of the diseases of the heart; aware of their existence, of their ailments, and of the deleterious complications and infirmities that ensure from them, and recognise that these diseases prevent one from attaining this courtesy. Knowledge of the diseases of the heart, their causes, and how to remove them is an obligation on every soul, every adult human being.
This whole matter points to the heart as a spiritual organ. The unseen aspect of the heart contains a seed that has the potential of becoming like a cancer that can metastasise and overtake the heart.
The analogy is that there is a dormant element in the human heart that, if nurtured and allowed to grow, can damage the soul and eventually bring its ruin.
The Prophet, Allah's blessings and peace be upon him, stated,
"If the son of Adam sins, a black spot appears in the heart. And if the person makes repentance, it is erased. But if he does not [repent], it continues to grow until the whole heart becomes pitch black."
Incidentally, this notion of associating the colour black with sin is not racist in its origins. The attribution has been long used among some black Africans who refer to a person who is wretched as "black-hearted".
The Qur'an says about successful people on the Day of Judgement that their faces become white (Qur'an, 3:106). This does not mean white as a hue of skin; rather it refers to light and brightness, which are spiritual descriptions not associated with actual colour.
A black person can have light in his face and a white person can have darkness and vice versa, depending on one's spiritual and moral condition.
Imam al-Ghazali considers ailments of the heart to be part of the Academic potential. he believes one is obliged to know this about human nature in order to be protected. Other scholars simply consider these ailments to be predominant in man; that is, most people have these potential qualities, but not necessarily everybody.
It is interesting that Imam Mawlud [the author, rahim-ullah] says it is impossible to rid oneself of these diseases without a trace. What this implies is that purification is a life-long process, not something that is applied once and then forgotten. Purity of heart never survives a passive relationship. One must always guard his or her heart.
There is a well known hadith that states that every child is born in the state of fitra. For some reason, Muslims often translate this into English as, 'Every child is a born a Muslim'. But the hadith says fitra, which means that people are born inclined to faith-born with an intuitive awareness of divine purpose and a nature built to receive the prophetic message.
What remains then is to nurture one's fitra and cultivate this inclination to faith and purity of heart.