Just as the heart may be described in terms of being alive or dead, it may also be regarded as belonging to one of the following three types:

The Healthy Heart 

On the Day of Resurrection, only those who come to God with a healthy heart will be saved. God says:

"The Day on which neither wealth nor sons will be of any use, except for whoever brings to God a sound heart." [Quran 26:88-89] 

In defining the healthy heart, the following has been said: "It is a heart cleansed from any passion that challenges what God commands, or disputes what He forbids. It is free from any impulses which contradict His good. As a result, it is safeguarded against the worship of anything other than Him, and seeks the judgment of no other except that of His Messenger . Its services are exclusively reserved for God, willingly and lovingly, with total reliance, relating all matters to Him, in fear, hope, and sincere dedication. When it loves, its love is in the way of God. If it detests, it detests in the light of what He detests. When it gives, it gives for God. If it withholds , it withholds for God. Nevertheless, all this will not suffice for its salvation until it is free from following, or taking as its guide, anyone other than His Messenger . A servant with a healthy heart must dedicate it to its journeys end and not base his actions and speech on those of any other person except God's Messenger . He must not give any precedence to any other faith, words or deeds over those of God and His Messenger. God says:

"O you who have attained to faith! Do not put yourselves forward  in the presence of [what] God and His Apostle [may have ordained], but remain conscious of God: for, verily, God is all-hearing, all-knowing! " [Quran 49:1] 

The Dead Heart 

This is the opposite of the healthy heart. It does not know its Lord and does not worship Him as He commands, in the way which He likes, and with which He is pleased. It clings instead to its lust and desires, even if these are likely to incur God's displeasure and wrath. It worships things other than God, and its love and its hatreds, and its giving and its withholding, arise from its whims, which are of paramount importance to it and preferred above the pleasure of God. Its whims are its imam. Its lust is its guide. Its ignorance is its leader. Its crude impulses are its impetus. It is immersed in its concern with worldly objectives. It is drunk with its own fancies and its love for hasty, fleeting pleasures. It is called to God and the akhira [day of judgment] from a distance but it does not respond to advice, and instead it follows any scheming, cunning Shaytan [Satan]. Life angers and pleases it, and passion makes it deaf and blind to anything except what is evil. To associate and keep company with the owner of such a heart is to tempt illness, living with him is like taking poison, and befriending him means utter destruction.

The Sick Heart

This is a heart with life in it as well as illness. The former sustains it at one moment, the latter at another, and it follows whichever one of the two manages to dominate it. It has love for God, faith in Him, sincerity towards Him, and reliance upon Him, and these are what give it life. It also has a craving for lust and pleasure, and prefers them and strives to experience them. It is full of self-admiration, which can lead to its own destruction. It listens to two callers: one calling it to God and His Prophet and the Hereafter; and the other calling it to the fleeting pleasures of this world. It responds to whichever one of the two happens to have most influence over it at the time. The first heart is alive, submitted to God, humble, sensitive, and aware; the second is brittle and dead; the third wavers between either its safety or its ruin.

Excerpted from: "Purification of the Soul" by Imam Abu Hamid Al-Ghazali