“In its biblical concept, the heart is the source of all the potentiality of the spiritual and physical life: ‘Keep your heart with all vigilance, for from it flows the springs of life’ (Prov. 4:23). This applies not only to good potentialities but to evil ones as well: ‘For our of the heart come evil thoughts, murder, adultery, fornication, theft, false witness, slander’ (Mt. 15:19). So the heart has become the expression of the final condition of man, whether he be good or evil. ‘The good man out of the good treasure of his heart produces good, and the evil man out of his evil treasure produces evil’ (Lk. 6:45). This means that the inclinations of the inmost heart set the tone of the whole man – they color his thoughts, his words, and his deeds. Man’s speech thus inevitably betrays the nature of his heart: ‘For out of the abundance of the heart, his mouth speaks’ (Lk. 6:45). So man’s words usually testify to the state of his heart. They can justify him or condemn him: ‘For by your words you will be justified, and by your words you will be condemned’ (Mt. 12:37). … St. Macarius the Great insists that the evil heart contaminates the will. It corrupts the natural inclinations and instincts of a man. Without his knowing it, everything that such a person sees and touches becomes impure for him:
So, on the contrary, as many as are sons of darkness, sin has control over their heart and infiltrates into all the members. ‘For out of the heart proceed evil thought’ (Mt. 15:19). And thus diffused throughout, sin covers man with darkness….
Just as water runs through a pipe, so also sin runs through the heart and the thoughts. All those who deny these statements are refuted and ridiculed by sin itself, which is always intent on victory. For evil tries to hide itself and remain undetected in man’s mind. (St. Macarius the Great, Homilies 15:21 in Spiritual Homilies.)
Hence, first among man’s struggles and concerns is to purify his heart. His endeavor is to overcome the deviations of the will and to correct the inclinations and instincts that have been subjected to the rule of evil. This means that he has to confront the tendency of his heart toward evil activity. He has to bridle it, curb it, and finally destroy this tendency.” (Matthew the Poor, Orthodox Prayer Life, pp 133 & 137)