The Jesus Prayer has been used for many centuries mostly in the Greek Orthodox Church as well as the Russian Orthodox Church. And while it is a simple prayer to learn and to use, it is a most powerful prayer. It's power of course comes from the name of Jesus, "that at the name of Jesus every knee should bow, in heaven and on earth and under the earth, and every tongue confess that Jesus Christ is Lord, to the glory of God the Father." Phil. 2:10-11. It was developed in the Hesychast tradition of prayer by monks of the Greek Orthodox Church on Mount Athos. Many Roman Catholics such as myself use the prayer daily to center ourselves and to try and remain in the presence of God to fulfill the command of St. Paul, "to pray always." I Thess 5:17 There are many writings concerning the Jesus Prayer in the various traditions. The Philokalia is such a book. It contains many writings of the early church fathers that explain and teach the Prayer.
What exactly then is the Jesus Prayer?
"Lord Jesus Christ, Son of God, have mercy on me a sinner." Some repeat this prayer while following the heartbeat. I have read many books that suggest using it with the breath instead. The theology behind the prayer is both profound and very simple. You cannot pray and declare that Jesus is Lord except in the Holy Spirit. So when you pray this prayer you are in effect allowing the Holy Spirit to pray within you the theological truth that Jesus is the source of everything that is good. I sometimes use the whole prayer in this way; on the intake of my breath I say (or think) the words, "Lord Jesus Christ, Son of God," and as I exhale I finish the prayer saying, "Have mercy on me, a sinner." This is said of course prayerfully and slowly, gently allowing the heart to speak to God. Writers like Thomas Merton have said that it is a perfect prayer in that the whole of theology is present in it. First we call Jesus the Lord. Actually we could probably stop there. We also are calling Jesus the Christ, thereby acknowledging his role as the Messiah and Saviour, and as the Son of God we remember his relationship in the Holy Trinity. After exalting the Lord in this manner filling our lungs with his presence and his Holy Name, we exhale reminding ourselves of our dependence on him by asking in all humility for His mercy and admitting what everyone knows in the depth of his being, that we are all sinners. This does not imply that it is a self-defeating attitude but one of hope that relies on the fact that while we are sinners the Lord still attends to our every prayer and to which we rely on Mercy and Forgiveness. Many times, I shorten the prayer somewhat to fit more easily to my pattern of breathing. The importance of the prayer lies in the name of Jesus. Sometimes I prefer, "Lord Jesus Christ, have mercy on me."