One of Kierkegaard's famous lines is that "purity of heart is to will one thing." Purity for him has to do with extracting from the heart all of the elements that are contrary to the one thing we should will, which he named the Good.
In the Sermon on the Mount, Jesus says, "The eye is the lamp of the body. So if your eye is sound, your body will be full of light; but if your eye is not sound, your whole body will be full of darkness. If then the light that is in you is darkness, how great is that darkness!" (Matthew 6:22-23).
Given these two ideas, one might conclude that to the degree that we have divided or fragmented hearts, our hearts are impure and our vision is obscured. Keeping our hearts sound is key not only to seeing ourselves as we really are, but to seeing God.
How do we go about having a pure heart? Jesus provides a clue in the next verse: "No one can serve two masters; for either he will hate the one and love the other, or else he will be loyal to the one and despise the other. You cannot serve God and mammon." Mammon is the idol of wealth, and all the minor desires, lusts, pleasures and interests that are compacted within the pursuit of money and riches. In order to have a pure heart, one must have one master, in order to will one thing.
What Is a Heart? The basic, historical, Christian definition of the heart is that it is the absolute center of the human person.
What is purity of heart? St. Isaac of Nineveh says that purity of heart "is a heart full of compassion for the whole of created nature..." He continues:
"And what is a compassionate heart? ... It is a heart which burns for all creation, for the birds, for the beasts, for the devils, for every creature. When he thinks about them, when he looks at them, his eyes fill with tears. So strong, so violent is his compassion ... that his heart breaks when he sees the pain and suffering of the humblest creature. That is why he prays with tears at every moment ... for all the enemies of truth and for all who cause him harm, that they may be protected and forgiven. He prays even for serpents in the boundless compassion that wells up in his heart after God's likeness."
If we regard the Beatitudes as a ladder, we arrive at purity of heart when we start with poverty of spirit, which among many other virtues is rooted in humility and leads to gratitude. Next, we mourn for our sins, the sins of others and the corruption that through the reality of death has overtaken all of existence and penetrated nature itself. We remain meek, refusing to be filled with notions of entitlement or to compare ourselves with others or with what other people have or what other people do. We hunger and thirst for righteousness as if it were bread and water, the righteousness of Christ that justifies us so that we may act justly in the world. We extend mercy to everyone for everything, not judging anyone for anything and in so doing we feed the hungry and visit the prisoner and give to the poor. We extend mercy and hospitality to whoever is put before us at any moment. And as we follow these steps, we also have the promise of the kingdom of heaven; we find comfort; the rift between our spirits and bodies begins to be healed and we are reconciled with the earth and the world of matter; we are filled with authentic righteousness and we obtain mercy for our own sins. The foregoing are the prerequisite steps to purity of heart, and one moves up the ladder through humility, watchfulness and prayer.
The word nepsis is a theological term that is used to denote watchfulness or mindfulness. Another translation is sobriety. An unsober heart need not be one that has been taken captive by wine, alcohol or other addicting substances, but it can be one that perhaps is intoxicated by pride, or lulled to sleep by discursive thoughts. A heart that is pulled into fragments by various lusts -- by the need to compare oneself with others, by expectations, by the feeling of entitlement, by envy or boredom or shame -- is a heart that is not sober.
By contrast, nepsis may also be thought of as mindfulness, awareness of oneself and of God's presence everywhere and in all things, a presence that, as our awareness enters into it, excludes many of our ambitions, worries, comparisons and judgments. Prayer is essential to watchfulness. Many people find the Jesus prayer ("Lord Jesus Christ, have mercy on me") to be useful in summoning this type of awareness and of entering into the stillness that is the reality of God's presence. One may also find it critical to follow the footsteps of others who have better vision, as well. Watchfulness, mindfulness and prayer that strengthens the will in cooperation with divine grace to will one thing -- communion with God -- unifies the heart and gradually makes it pure.
There are many saints in the Orthodox Church who exemplify purity of heart, but one of the most popular in the Russian Church and around the world is St. Seraphim of Sarov. Jim Forest writes succinctly about St. Seraphim in his book, The Ladder of the Beatitudes:
"In talks with visitors Saint Seraphim stressed "the acquisition of the Holy Spirit" so that the kingdom of God can take possession of the heart. A man of constant prayer and fasting, Seraphim reminded them that ascetic practice was only a means to a greater end: 'Prayer, fasting, watching may be good in themselves; yet it is not in these practices alone that the goal of our Christian life is found, though they are necessary means for its attainment. The true goal consists in our acquiring the Holy Spirit of God.' On occasion he put the message even more simply: 'Acquire the Spirit of Peace, and thousands around you will be saved.'
Seraphim recognized kindness, joy, and the refusal to condemn others as signs of God's presence in the heart:
'You cannot be too gentle, too kind. Shun even to appear harsh in your treatment of each other. Joy, radiant joy, streams from the face of him who gives and kindles joy in the heart of him who receives. All condemnation is from the devil. Never condemn each other. We condemn others only because we shun knowing ourselves. When we gaze at our own failings, we see such a swamp that nothing in another can equal it. That is why we turn away, and make much of the faults of others. Instead of condemning others, strive to reach inner peace. Keep silent, refrain from judgment. This will raise you above the deadly arrows of slander, insult, and outrage and will shield your glowing hearts against all evil.'"
We are called to purity of heart, which is the heart cleansed by divine love, a love that washes the noetic lens so that we can see more clearly ourselves as we really are and as we are meant to become -- so that we can see the light that radiates from the very face of God, as well. This is our purpose. As we Orthodox sing in the service of Paschal Matins, our candles lit against the darkness that surrounds us in the midnight hour: "By Thy Resurrection O Christ our savior, the angels in Heaven sing, enable us who are on Earth, to glorify thee in purity of heart."