Purification of the souls is one of the ends for which the prophets were sent. The true believer is the one who does not harbor any grudge against anyone, nor does he look for others’ faults. A clean heart is an indication of perfect belief and of being a good Muslim. However, whoever does otherwise is not fit for Heaven to be his abode, as his heart is full of impurities.
Bravery, courage, generosity, love, kindness, compassion, Allah-consciousness, altruism, nearness to Allah and enjoying having intimate discourse with Allah, belief, certitude, and all kinds of good cannot exist except in a pure heart. They cannot exist in a heart polluted with malice, envy, selfishness, and ill will toward others. The pious people are always keen to watch their hearts and purify them. The pure hearts and virtuous souls are the ones that are filled with Allah-consciousness, being fearful of Allah, and having strong belief, so they overflow with good and kindness. People with such hearts are characterized by upright conduct and their inner self is pure. Such people love to do good to others as they themselves are living in peace.
As to the person with an evil heart and bad manners, people suffer because of their evil. Also, such people have suffering souls. A good Muslim does not bear any grudge or hatred against his brethren. The closest persons to the Prophet (peace be upon him) were those with excellent manners, morals, and pure and sincere hearts. Such hearts and souls take the persons to the highest levels in Paradise, and they wouldn’t have reached these levels even with the voluntary (nawafil) prayers.