In this world man has to lead his life in circumstances in which he experiences various kinds of situations: there are times of gain, times of loss; times of happiness and times of grief. Sometimes he receives good treatment at the hands of others, at other times his fate is quite otherwise.

The ideal human being of the Quran is one who undergoes all these experiences without losing his integrity. Under no circumstances is his inner peace disturbed. However, untoward the occasion, he can maintain his natural balance. Success does not make him proud. Power does not make him haughty. No bad treatment by others drives him to seek vengeance in anger. At all events, he remains serene. It is such a man who is called 'a peaceful soul' in the Quran. And it is this man who, according to the Quran, has achieved the highest spiritual state.

The realization of God joins man with his Maker. Such communion with the divine brings about a state of spiritual elevation. Having been thus raised to a higher plane of existence, man becomes of a 'sublime character,' (68:4) as it is expressed in the Quran.

Source: The True Face of Islam

Tazkiyah literally means purification. In the extended sense, it means growth. In this respect, tazkiyah means intellectual development or positive personality development.

The mind is not something stagnant, it is ever-growing, like a tree. It is this process that is called “adding faith to their faith” (48:4) in the Quran. Adding faith to faith means development in consciousness, which is another name for intellectual development. The real faith is one which does not stagnate but grows continuously in its conviction about God.

How does tazkiyah or intellectual development take place? The path to this is contemplation. Contemplation is in itself a continuous process—contemplating the Quran and Hadith, the life of the Prophet, the lives of the Companions, the various human disciplines, the universe, the earth, and the heavens. There is also the kind of contemplation that takes place during serious discussions. In this process of contemplation, new ideas emerge, deeper meanings come to light, new aspects hitherto unexplored of different events and realities are discovered, etc.

For one who possesses true faith, every study and observation will become a means of divine discovery; every experience will bring him closer to God. If initially, his faith is in the form of a seed, it will keep growing and assume the form of a fully-grown tree—tazkiyah is the Islamic name for this intellectual and spiritual process.

Faith initially brings us into the fold of Islam, then tazkiyah helps in the development of this faith.

Source: The True Face of Islam

The attainment of tazkiyah is wholly a psychological act and can be attained only at that level.  A psychological act means an act at the intellectual level. The human mind is at the centre of all kinds of human development. It is in fact the mind which is the deciding factor in shaping the human personality. The non-purified personality and the purified personality are both products of the mind.

It is a positive intellectual development that is actually required for tazkiyah. That is, one has to develop one’s conscious mind to such an extent that it may become discriminating, it may convert negative thoughts into positive thoughts, it may have glimpses of the Creator in His creation, and may discover spiritual aspects in material events.

It may reject Satan’s temptations, it may rise above the temptations of the self, it may distance itself from fruitless actions, it may make out its real well-wishers, it may accept advice, even if that be against its nature, it may develop anti-self-thinking, and be able to set the goal of spirituality for itself, rising above material goals. One who thinks along these lines will become hereafter-oriented.

All these developments take place at the psychological level, being the result of deep thinking. One who cannot engage in deep thinking will never attain high levels of tazkiyah. The process of tazkiyah can be initiated only in one who is capable of deep thought. Tazkiyah, in fact, is another name for psychological purification. First of all, tazkiyah is performed at the psychological level and only then is it possible for tazkiyah to be achieved at the level of the whole human existence.

Source: Tazkiyah: Purification of the Soul

The body requires physical food. When it receives its food, it becomes healthy. Similarly, the soul requires spiritual provision. When the soul is provided with such food, it becomes healthy. This process is called purification of the soul and this healthy soul is called a purified and cleansed soul.

According to the Quran, the food for the soul is in thinking. (3:191). At every moment certain incidents take place in the life of a man. These incidents appear at the level of society, history, the universe, etc. Thinking about these incidents or events and deriving lessons from them, is food for the soul.

One who develops his consciousness to such an extent that he sees the glory of God in the events which take place around him, and for whom these events become a means of remembering God, has obtained the divine provision for his soul. His soul will continue to receive this healthy nourishment until he finally leaves this world to be ushered into the Divine Presence.

The most important source of tazkiyah is the lessons one learns to take from every happening. The ability to learn lessons is the basis of tazkiyah. This is the soil from which tazkiyah grows. Trying to encourage its growth at any other place is like trying to grow a green tree on a rock. The source of tazkiyah is divine provision and not human provision. It is the result of that process which is directly established between God and man through a psychological relationship. Tazkiyah is not achieved through any intermediary between God and man. Tazkiyah is a blessing which man receives from God.

Source: Tazkiyah: Purification of the Soul