What is not in the heart is not in the thing itself: faith is not faith; love is not love, when it is not in the heart, but it is hypocrisy; humility is not humility, when it appears only outwardly, but has no place in the heart. God requires from us our heart: My son, give me thine heart (Proverbs 2326).
The sun is seen clearly in pure and still water, and its likeness is depicted therein; so also does God, the eternal Sun, appear in a still, blameless and pure soul, and depicts His image therein. Having therefore these promises, dearly beloved, let us cleanse ourselves from all filthiness of the flesh and spirit, perfecting holiness in the fear of God (II Corinthians 7:1), that God the eternal Sun will also dwell in us and depict in our soul His holy image.
Hierarch Tychon of Zadonsk
A good man out of the good treasure of his heart bringeth forth that which is good; and an evil man out of the evil treasure of his heart bringeth forth that which is evil: for of the abundance of the heart his mouth speaketh (Luke 6:45). That is why the Apostle commands us also: Put on therefore, as the elect of God, holy and beloved, bowels of mercies, kindness, humbleness of mind, meekness, longsuffering (Colossians 3:12). Put on, that is, impress these dispositions on thy soul.
Bishop Theophan, the recluse of Vysha
Remove from thy heart pride, which is an abomination before God, acknowledge thy spiritual poverty, turn to God with a heartfelt cry for help against the unclean thoughts that bestorm thy heart and the Allpowerful Lord will grant thee the hand of His gracious help and will free thee from the spiritual defilements cast upon thee by the enemy of our salvation the devil; He will purify thy heart and make it a temple for Himself.
Abba Dorotheus
Create in me a clean heart, O God, and renew a right spirit within me. Cast me not away from Thy presence, and take not Thy Holy Spirit from me (Psalm 50:1213).