Purification of the heart

Chapter 26 of the Quran tells us that only those will be held deserving of Paradise in the Hereafter who come to God with a sound heart. (26:89) In this verse, “heart” does not indicate an organ, in the physiological sense. It means rather the psyche.
Zahhak, the well known commentator has explained qalb (‘sound heart’) to mean ‘pure heart’. (Tafsir al-Qurtubi, vol. 13, pp. 114). That is, an uncorrupted heart. One who guards his nature of impurities, by successfully combating conditioning of his environment and thus succeeds in deconditioning his mind, will attain to God-realization. Such persons alone will be saved who come to God with a sound heart. (The Quran, 26:89)
The truth is that everyone is born with an upright nature. But when they fall under the influence of their environment, their nature systematically becomes tainted. To free oneself from external influences and restore one’s nature to its original state, one must engage in introspection. This objective introspection guarantees that a man will return to having an upright nature or a sound heart. Those who reach God with a sound heart will be held deserving of being ushered into the eternal world of Paradise. (The Quran, 26:89)
A sound heart or sound mind can be achieved by an intellectual struggle. One who engages himself in such a struggle becomes serious in the best sense of the word.
He develops the ability to discover the truth in a jungle of ideologies and conflicting information. He develops the ability to analyze all such information with total objectivity. He is able to look at things from the right angle. This is called having a sound heart or unimpaired judgement. This high level of ratiocination is achieved through great striving on a high intellectual plane.