Purification of the self (nafs) or the soul (rooh) means to prepare it to receive, recognize, and reflect the Truth, namely Allah, the Creator and the Sustainer of all existence. Purification of the soul, therefore, is the purpose of all acts of worship. Allah Almighty says that the purpose of the mission of the Prophet was to purify and teach us.
He Almighty Says (what means): {As we have sent to you a messenger from among you who recite to you Our verses, and purifies you, and teaches you the Book and wisdom, and teaches you what you knew not.} [Quran 2:151]
The truth needs a ready receptacle. If the human receptacle of truth, which is referred to in the Quran and the Sunnah as the heart, is not ready for it, the truth of God never settles deep in it. The reception of truth has been likened by Allah Almighty and the Prophet to the coming down of rain to the earth.
The Prophet said: "The example of guidance and knowledge with which Allah has sent me is like abundant rain falling on the earth, some of which was fertile soil that absorbed rain water and brought forth vegetation and grass in abundance. (And) another portion of it was hard and held the rainwater and Allah benefited the people with it and they utilized it for drinking, making their animals drink from it and for irrigation of the land for cultivation. (And) a portion of it was barren which could neither hold the water nor bring forth vegetation (then that land gave no benefits). The first is the example of the person who comprehends Allah's religion and gets benefit (from the knowledge) which Allah has revealed through me, and learns and then teaches others. The last example is that of a person who does not care for it and does not take Allah's guidance revealed through me (He is like that barren land.)" [Al-Bukhari]
Some land is fertile, and upon receiving rain, gives out vegetation. With some human effort and planning, the fertile land turns into beautiful, refreshing, life-giving and life-sustaining gardens and farms. Some land can faithfully hold water to be used by others. Other land is not ready for the benefit of rain, and upon receiving water, it may turn into deadly marshes and swamps, or it is so barren and hard that the water runs off of its surface, depriving it further of any nutrients.
Just the same way, there are many Muslim (one may even say the vast majority) who even after receiving the truth of Islam, remain barren of any of its fruit. Yet, there are always those who do receive and bloom even if their share of the knowledge of the truth is small, but their fertility is high, because they have clean receptacles.
There are memorizers of the Quran, graduates of Islamic universities, and highly revered Shaykhs and so-called spiritual masters, big and small, whose receptacles are unclean and their knowledge becomes only a burden against them. Some of them know little but want to be revered as great Shaykhs.
Some have memorized a lot, but internalized and practiced very little, whose hairsplitting has left no room for Allah's love and fear in their hearts.
Some have a great amount of knowledge and impressive books and speeches to their credit, but whose self-love has made their knowledge into a beneficial trade, a mark of honor. They are not willing to sacrifice their positions for the truth.
Some have all the outer accoutrements of piety and worship, from prayers to fasting, but who keep their piety separate from their worldly status. They perform Hajj often and sponsor Mosques and charities, but their businesses may deal in interest or alcohol or other prohibitions. As the Prophet said: "I know of people in my Ummah who will come on the Day of Resurrection with good deeds like the mountains of White Tihama, but Allah will turn them into worthless dust. They are your brothers, from your skin, and they take from the night (of worship) what you do. But they are people who when they are alone with God's prohibitions, they violate them." (Graded as saheeh by Al-Albaani)
Some are imams and Khateebs, known as scholars or Shaykhs or leaders of this institution or that. Yet, when they are tested they show all the signs of hypocrites: As the Prophet said: "Signs of a hypocrite are three. When he speaks, he lies. When he makes a promise, he breaks it. When he is trusted, he violates the trust." [Al-Bukhari and Muslim] Another narration adds another sign, "When he disagrees, he abuses and insults." [Al-Bukhari and Muslim] The narration of Muslim adds, "Even if he fasts, prays, and claims to be Muslim."
The way to prepare our hearts for the truth is to be truthful: To love truth, advocate truth, speak the truth even at a great cost, and be truthful to ourselves about our weaknesses, needs, feelings and strengths, and finally, to be truthful to Allah Almighty.