By Fr. George Morelli 
In the tradition of the Eastern Church the heart is the true center of our being. The Psalmist (9:1) tells us: "I will give thanks to the LORD with my whole heart; I will tell of all thy wonderful deeds." In our Orthodox tradition, which is the teaching of Jesus passed on to His Apostles and guided by the Holy Spirit, the focal point of all honesty of our relationship with God and with our neighbor is our hearts. In other words it is all about heart. Think of some of the words of Jesus on this: "Blessed are the pure in heart, for they shall see God” (Mt 5:8). To the scribes who condemned Jesus as a blasphemer for forgiving the sins of the paralytic St. Matthew (9:4) records that ". . . Jesus, knowing their thoughts, said, “Why do you think evil in your hearts." Jesus also told his listeners “. . . but I say to you that everyone who looks at a woman with lust for her has already committed adultery with her in his heart” (Mt 5:28), and “for where your treasure is, there your heart will be also” (Mt 6:21). And, speaking to the hypocrites, Jesus said: "You brood of vipers, how can you, being evil, speak what is good? For the mouth speaks out of that which fills the heart. (Mt 12:34). And again: " That which proceeds out of the man, that is what defiles the man” (Mk 7:20).
Focus on the heart is not limited to Eastern Christian tradition. The Islamic mystic, Rumi (c.1244 AD), wrote: "When your heart is dark as iron, steadily polish yourself that the heart may become a mirror, a beautiful shine reflecting from within. Although iron is dark and dismal, polishing clears the darkness away."[i] The Hindu Upanishad states: "The Self is hidden in the lotus of the heart. . . .Those who know this live day after day in heaven in this very life."[ii]
Our Eastern Church Father, St. Cyprian of Carthage tells us: “God listens, not to our voice, but to our heart. He does not need to be prodded with shouts, since He sees our thoughts, as the Lord proved when He asked, "Why do you think evil in your hearts?" And elsewhere He stated: "All the assemblies shall know that I am He who searches minds and hearts." (Rev 2: 23).[iii] What are the implications for us of Jesus’ teaching on the heart? First, we may fool ourselves or others, but we can never fool God. Once again, duplicity was certainly known to the Hebrews: "Every one utters lies to his neighbor; with flattering lips and a double heart they speak." (Ps.11:  2). The psalmist King David noted: "If I regard wickedness in my heart, The Lord will not hear. . . ." (Ps 65: 18). Second, God can see if we are holding back in any way, that is to say, as the Native American proverb goes: ‘If we are speaking with forked tongues.’[iv] This insight is made more spiritually fruitful by St. John Chrysostom[v] who tells us: “It is not enough to show mercy, but it behooves us to do it with openhandedness and an ungrudging spirit. . . even with a cheerful and rejoicing one.” Now St. John was talking about mercy, but we can extend this openhandedness, cheerfulness and non-reluctance to doing, from the depths of our hearts, anything and everything that has a spiritual dimension; this would mean: love; forgiveness; kindness; generosity. Think about it, however, mercy encompasses both the Spiritual and Corporal Works of Mercy. So “mercy” as used by St. John Chrysostom encompasses all: admonishing sinners, instructing the ignorant, counseling the doubtful, comforting the sorrowful, suffering wrongs patiently, forgiving injuries, and praying for the living and the dead; also, feeding the hungry, giving drink to the thirsty, clothing the naked, ransoming captives, sheltering the homeless, visiting the sick and burying the dead. All the above can be done out of duty, with grumping and moaning, for admiration or expecting payback (Morelli, 2007). From the heart it has to be done solely with the love God expects of us.
Finally, we can keep in mind the wisdom of one the greatest spiritual Church Fathers of the Eastern Church, St. Isaac of Syria (Wensinck, 1923): "Purity of mind is something other than purity of heart, just as there is a difference between one of the members of the whole body and the whole body. The mind is one of the senses of the soul. The heart is the central organ of the inward senses, because it is the root. And if the root is holy so are all the branches. But this is not so if it is holy in one of the branches only."
Morelli, G. (2007, April 27). Good Marriage II. Reciprocity—The One-Way Contract that can Wreck a Marriage. http://www.orthodoxytoday.org/OT/view/good-marriage-II-reciprocity-the-one-way-contract-that-can-wreck-a-marriage.
Wensinck, A.J. (1923). (ed., trans.), Mystic Treatises by Isaac of Nineveh. Amsterdam, Holland: Koninklijke Akademie Van Wetenschappen.
V. Rev. Fr. George Morelli Ph.D. is a licensed Clinical Psychologist and Marriage and Family Therapist, Chairman of the Dept. of Chaplaincy and Pastoral Counseling of the Antiochian Orthodox Christian Archdiocese , (http://www.antiochian.org/counseling-ministries ) and Religion Coordinator (and Antiochian Archdiocesan Liaison) of the Orthodox Christian Association of Medicine, Psychology and Religion . Fr. George is Assistant Pastor of St. George's Antiochian Orthodox Church, San Diego, California.