The Prayer of the Heart, also known as The Jesus Prayer, is a very simple, one line prayer: “Lord Jesus Christ, Son of God, have mercy on me.” Some versions of the prayer add “a sinner” to the end of the prayer, but that is a later addition. The prayer’s history is long and complex, but can be traced all the way back to the Desert Fathers of Egypt in the early centuries of the church. The prayer was made popular by the book, ‘The Way of the Pilgrim,’ written by an anonymous Russian contemplative in the mid 19th century.
Practicing the Prayer of the Heart requires a quiet place where one can be alone and undistracted. Once the external setting is determined, the real work begins. Teachers of the Prayer of the Heart describe the posture required for the prayer as allowing “the mind to descend into the heart with the breath.” While this may seem like a perplexing instruction, it simply means to allow your mind to empty itself of thoughts, to quiet the ego, and to be in a posture of open receptivity to God. Once you are settled into this posture, allow yourself to repeat the prayer with the rhythm of your breath. Breathing in: “Lord Jesus Christ, Son of God;” breathing out: “have mercy on me.”
To aid in this prayer, it is often recommended to use prayer beads or a knotted prayer rope, to keep track of the number of repetitions.
For further reading: The Way of the Pilgrim, anonymous; Breath Prayer: An Ancient Practice for the Everyday Sacred, by Christine Valters Paintner