What is the Jesus Prayer?|
For the complete fulfillment of its [the mind's] purpose we should give it nothing but the prayer 'Lord Jesus'...
St. Diadochos of Photiki (d. 486)
(1) Lord Jesus Christ, have mercy on me. (2) Lord Jesus Christ, Son of God, have mercy on me, a sinner
The first is used at Mt. Athos and seen in The Way of the Pilgrim; the second is the most popular version.
...the grace-given prayer of the mind in the heart, which shuns imaginings, "is not characteristic of children; wherefore we who are like children, being concerned with the perfection of our prayer," that is, the attentiveness which is acquired by locking the mind into the words of the prayer, "must pray a great deal. Quantity is the cause of quality. The Lord gives pure prayer to him who, eschewing laziness, prays much and regularly in his own manner, even if it is marred by inattention".
St. Ignatius Brianchaninov (d. 1867) quoting The Ladder of St. John of the Ladder (d. 606)
There is yet another form of prayer, which is called standing in the presence of God, when the man who prays is wholly concentrated in his heart and inwardly contemplates God as being present to him and within him, with corresponding feelings...Such a state comes when a man becomes deeply immersed in prayer by word, mind and heart. If a man prays in the right way and for a long time, these states come to him more and more often, and finally this state can become permanent; then it is called walking before God and is constant prayer.
Lorenzo Scupoli (d. 1610) in Unseen Warfare edited by St. Theophan the Recluse (d. 1894)