The Jesus Prayer, also called the Prayer of the Heart by some Church Fathers, is a short, simple prayer that has been widely used, taught and discussed throughout the history of Eastern Christianity. The exact words of the prayer have varied, from a simple form such as "Lord, have mercy" to the more common extended form: "Lord Jesus Christ, Son of God, have mercy on me, the sinner." The form most in use on Mount Athos is "Lord Jesus Christ, Son of God, have mercy on me."
It is, for the Orthodox, one of the most profound and mystical prayers and is often repeated endlessly as part of a personal ascetic practice. There have been a number of Roman Catholic texts on the subject, but its usage has never achieved the same degree of devotion as in the Eastern Church. A more elaborate version known to some Roman Catholics by the same name goes: "O my Jesus, forgive us our sins, save us from the fires of Hell, and lead all souls to Heaven, especially those most in need of Your mercy."
The prayer is most reflective of the lesson taught by the parable of the Publican and the Pharisee; in which the Pharisee demonstrates the improper way to pray by exclaiming, "Thank you Lord that I am not like the Publican." While the Publican in humility prays correctly “"Lord have mercy on me, the sinner" (Luke 18:10-14). And likewise in the Gospels, Peter crying out as he sank into the sea, "Lord, save me."
In the Orthodox tradition the prayer is said or prayed repeatedly, often with the aid of a prayer rope. It may be accompanied by prostrations and the sign of the cross. As such, it is used as a means of finding contrition and as a means of bringing about humility in the individual; hence the words "the sinner" are sometimes added as if no other sinner existed but the person praying (though there is no indefinite article in Greek, thus leading to some controversy about whether the translation in English should be "the sinner" or "a sinner"). Monastics often have long sessions praying this prayer many hundreds of times each night as part of their discipline, and through the guidance of an elder, its practitioner’s ultimate goal is to "internalize" the prayer, so that one is praying unceasingly there-by accomplishing Saint Paul's exhortation to the Thessalonians to "pray without ceasing" (I Thessalonians 5:17). The use of the Jesus Prayer in this way is the subject of the Russian classic The Way of a Pilgrim.