In order to enter more deeply into the life of prayer and to come to grips with St. Paul's challenge to pray unceasingly, the Orthodox Tradition offers the Jesus Prayer, which is sometimes called the prayer of the heart. The Jesus Prayer is offered as a means of concentration, as a focal point for our inner life. Though there are both longer and shorter versions, the most frequently used form of the Jesus Prayer is: 

"Lord Jesus Christ, Son of God, have mercy on me, a sinner." 

This prayer, in its simplicity and clarity, is rooted in the Scriptures and the new life granted by the Holy Spirit. It is first and foremost a prayer of the Spirit because of the fact that the prayer addresses Jesus as Lord, Christ and Son of God; and as St. Paul tells us, "no one can say 'Jesus is Lord' except by the Holy Spirit" (1 Cor. 12:3).

THE SCRIPTURAL ROOTS OF THE JESUS PRAYER
The Scriptures give the Jesus Prayer both its concrete form and its theological content. It is rooted in the Scriptures in four ways:

1. In its brevity and simplicity, it is the fulfillment of Jesus' command that "in praying" we are "not to heap up empty phrases as the heathen do; for they think that they will be heard for their many words. Do not be like them . . . (Matt. 6:7-8).

2. The Jesus Prayer is rooted in the Name of the Lord. In the Scriptures, the power and glory of God are present in his Name. In the Old Testament to deliberately and attentively invoke God's Name was to place oneself in his Presence. Jesus, whose name in Hebrew means God saves, is the living Word addressed to humanity. Jesus is the final Name of God. Jesus is "the Name which is above all other names" and it is written that "all beings should bend the knee at the Name of Jesus" (Phil. 2:9-10). In this Name devils are cast out (Luke 10:17), prayers are answered (John 14:13 14) and the lame are healed (Acts 3:6-7). The Name of Jesus is unbridled spiritual power.

3. The words of the Jesus Prayer are themselves based on Scriptural texts: the cry of the blind man sitting at the side of the road near Jericho, "Jesus, Son of David, have mercy on me" (Luke 18:38); the ten lepers who "called to him, Jesus, Master, take pity on us' " (Luke 17:13); and the cry for mercy of the publican, "God, be merciful to me, a sinner" (Luke 18:14).

4. It is a prayer in which the first step of the spiritual journey is taken: the recognition of our own sinfulness, our essential estrangement from God and the people around us. The Jesus Prayer is a prayer in which we admit our desperate need of a Saviour. For "if we say we have no sin in us, we are deceiving ourselves and refusing to admit the truth" (1 John 1:8).

THE THREE LEVELS OF PRAYER  

Because prayer is a living reality, a deeply personal encounter with the living God, it is not to be confined to any given classification or rigid analysis. However, in order to offer some broad, general guidelines for those interested in using the Jesus Prayer to develop their inner life, Theophan the Recluse, a 19th century Russian spiritual writer, distinguishes three levels in the saying of the Prayer:

1. It begins as oral prayer or prayer of the lips, a simple recitation which Theophan defines as prayers' "verbal expression and shape." Although very important, this level of prayer is still external to us and thus only the first step, for "the essence or soul of prayer is within a man's mind and heart."

2. As we enter more deeply into prayer, we reach a level at which we begin to pray without distraction. Theophan remarks that at this point, "the mind is focused upon the words" of the Prayer, "speaking them as if they were our own."

3. The third and final level is prayer of the heart. At this stage prayer is no longer something we do but who we are. Such prayer, which is a gift of the Spirit, is to return to the Father as did the prodigal son (Luke 15:32). The prayer of the heart is the prayer of adoption, when "God has sent the Spirit of his Son into our hearts, the Spirit that cries 'Abba, Father!'" (Gal. 4:6).

THE FRUITS OF THE JESUS PRAYER
This return to the Father through Christ in the Holy Spirit is the goal of all Christian spirituality. It is to be open to the presence of the Kingdom in our midst. The anonymous author of The Way of the Pilgrim reports that the Jesus Prayer has two very concrete effects upon his vision of the world. First, it transfigures his relation ship with the material creation around him; the world becomes transparent, a sign, a means of communicating God's presence. He writes:

"When I prayed in my heart, everything around me seemed delightful and marvelous. The trees, the grass, the birds, the air, the light seemed to be telling me that they existed for man's sake, that they witnessed to the love of God for man, that all things prayed to God and sang his praise." 

Second, the Prayer transfigures his relationship to his fellow human beings. His relationships are given form within their proper context: the forgiveness and compassion of the crucified and risen Lord.

"Again I started off on my wanderings. But now I did not walk along as before, filled with care. The invocation of the Name of Jesus gladdened my way. Everybody was kind to me. If anyone harms me I have only to think, 'How sweet is the Prayer of Jesus!' and the injury and the anger alike pass away and I forget it all." 

ENDLESS GROWTH
"Growth in prayer has no end," Theophan informs us. "If this growth ceases, it means that life ceases." The way of the heart is endless because the God whom we seek is infinite in the depths of his glory. The Jesus Prayer is a signpost along the spiritual journey, a journey that all of us must take.

http://www.goarch.org/en/ourfaith/articles/article7104.asp