A distinctive spiritual practice in Orthodox Christianity is the constant repetition of the “Jesus Prayer”: Lord Jesus Christ, Son of God, have mercy upon me, a sinner.
St. Theophan the Recluse, a popular spiritual writer in 19th century Russia, wrote:
The practice of the Jesus Prayer is simple. Stand before the Lord with the attention in the heart, and call to Him: “Lord Jesus Christ, Son of God, have mercy on me!” The essential part of this is not in the words, but in faith, contrition, and self-surrender to the Lord. With these feelings one can stand before the Lord even without any words, and it will still be prayer.
The practice of the Jesus Prayer in its present form stretches all the way to the fifth-century Greek bishop, St. Diadochos of Photiki, although the same principles are evident in even earlier spiritual writers, East and West.
His Eminence Metropolitan Kallistos Ware has written extensively on the subject and is one of the main translators into English of the Philokalia, the most important collection of monastic wisdom in the Orthodox East, whose authors often discuss the Jesus Prayer.
His Eminence delivered the lecture embedded above in June 2012 at St. John the Baptist Russian Orthodox Cathedral in Washington, D.C. Please note: the audio quality improves significantly about four minutes into the lecture. Patience is amply rewarded.