Why is a formulaic version of the Jesus Prayer (dating back to the 5th century) included in the Book of Mormon?
There is a very interesting formulaic prayer parallel between Alma 19:29 and the Jesus Prayer as well as the words of Our Lady of Fatima:
And it came to pass that she went and took the queen by the hand, that perhaps she might raise her from the ground;
and as soon as she touched her hand she arose and stood upon her feet, and cried with a loud voice, saying: O
blessed Jesus, who has saved me from an awful hell! O blessed God, have mercy on this people!
The Jesus Prayer: Lord Jesus Christ, Son of God, have mercy on me, a sinner.
"Oh my Jesus, forgive us our sins, save us from the fires of hell; lead all souls to heaven,
especially those most in need of thy mercy." Our Lady of Fatima, 1917
The Book of Mormon and Our Lady of Fatima use two common formulas, one a prayer of petition "Jesus/God save me/us from __________ and the second from part of the Jesus Prayer "have mercy on __________." The focus of this coupling is to first ask God to help us personally, to save us from something, and then to extend the petition making it possible for the first part to be accomplished through the granting of mercy to us or to others, since without mercy we would be unworthy to ask God to save us.
In medieval Europe a common prayer of many mothers when they put their children to bed at night was "Father, protect our springs of life and save us from the Huns".
Many scriptures may have been used as the basis for this form of formulaic prayer:
Now therefore, O LORD our God, save us from his hand, that all the kingdoms of the earth may know that thou art the LORD, even thou only.
Save us, O LORD our God, and gather us from among the heathen, to give thanks unto thy holy name, and to triumph in thy praise.
And he cried, saying, Jesus, thou son of David, have mercy on me.
And when Jesus departed thence, two blind men followed him, crying, and saying, Thou son of David, have mercy on us.
Lord Jesus Christ, Son of God, have mercy on me a sinner.
“The Jesus Prayer, also called the Prayer of the Heart by some Church Fathers, is a short, formulaic prayer often uttered repeatedly. It has been widely used, taught and discussed throughout the history of Eastern Christianity. The exact words of the prayer have varied from the most simple possible involving the name 'Jesus,' such as 'Lord have mercy,' to the more common extended form: 'Lord Jesus Christ, Son of God, have mercy on me, a sinner.'
“The Jesus Prayer is, for the Eastern Orthodox and Eastern-Rite Catholics, one of the most profound and mystical prayers and it is often repeated continually as a part of personal ascetic practice. Its practice is an integral part of Hesychasm, the subject of the Philokalia, a collection of texts on prayer compiled in the late 18th Century. There have been a number of Roman Catholic texts on the Jesus Prayer, but its practice has never achieved the same popularity as in the Orthodox Church. Moreover, the Eastern Orthodox theology of the Jesus Prayer, enunciated by St Gregory Palamas (1296–1359), has never been fully accepted by the Roman Catholic Church (see, e.g., Pope John Paul II's Angelus Message, August 11, 1996).
“The practice of repeating the prayer continually dates back to at least the fifth century. The earliest known mention is in the Gnostic Chapters of Saint Diadochos of Photiki (400-486), a work found in the first volume of the Philokalia. The Jesus Prayer is described in the Gnostic Chapters in terms very similar to St John Cassian's (?–435) description in the Conferences 9 and 10 of the repetitive use of a passage of the psalms. St Diadochos ties the practice of the Jesus Prayer to the purification of the soul. He also teaches that repetition of the prayer produces inner peace. The use of the Jesus Prayer is recommended in the Ladder of Divine Ascent of St John of Sinai (523–603) and in the work of St Hesychios (?8th Century), Pros Theodoulon, found in the first volume of the Philokalia. The use of the Jesus Prayer according to the tradition of the Philokalia is the subject of the Russian classic The Way of a Pilgrim, itself a major subject of J.D. Salinger's novel, Franny and Zooey.” (Wikipedia, http://en.wikipedia.org/wiki/Jesus_Prayer)
I believe the simple Jesus Prayer from early Christianity influenced the choice of words Joseph Smith used in his fictional Book of Mormon.