You cannot approach God without constant prayer. St Isaac the Syrian (d. 700) in Writings from the Philokalia
...it is the duty of all Christians, small and great, always to practice the mental prayer Lord Jesus Christ, have mercy on me, so that their mind and heart may acquire the habit of always uttering those holy words. Let this convince you how pleasing this is to God and what great good derives from it, since He, out of His infinite love for men, sent a heavenly Angel to tell us this, so that no one should have any doubt about it.
St. Gregory Palamas (d. 1359) relating an Angel of God's revelation in the Life of St. Gregory of Salonica
I made him (unruly godson) sit down on a bench next to me and I ordered him to say the Jesus Prayer without stopping...After some time I noticed the whip was no longer necessary, for the boy recited the Prayer willingly and diligently and I noticed a change in his behavior...he got so accustomed to the the Prayer that he was saying it no matter what he was doing...
Told to the anonymous author of The Way of a Pilgrim (19th century Russia); chapter 4
By it the heart is opened to human wretchedness and the Savior's mercy. The invocation of the holy name of Jesus is the simplest way of praying always. When the holy name is repeated often by a humbly attentive heart, the prayer... "brings forth fruit with patience. This prayer is possible "at all times" because it is not one occupation among others but the only occupation: that of loving God, which animates and transfigures every action in Christ Jesus.
1994 Catechism of the Catholic Church sections 2667-2668
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)
Lord Jesus Christ, have mercy on me. And, 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)
When Should We Pray the Jesus Prayer?Whatever work you are doing, or office you are holding, or journey you are going, do not cease to chant this. When you are going to bed, or eating, and in the last necessities of nature, think on this. This thought in your heart may be to you a saving formula...
St. John Cassian (d. 435) on repeating O God, make speed to save me: O Lord, make haste to help me
Set a definite length of time for prayer—a quarter of an hour, a half, or a whole hour (whatever is convenient), and regulate your vigil so that the clock striking on the half hour or the hour signals the end of prayers. Then when you begin prayers, do not concern yourself with the number of prayers read, but only lift your heart and mind to the Lord in prayer, and continue in a worthy manner for the time set aside. Others determine how many prayers can be done on the prayer rope in a given time and proceed in a calm and unhurried way to count them on the prayer rope.
Letters of St. Theophan the Recluse (d. 1894)
... ~ When you get up from your sleep, make the sign of the Cross over yourself three times with the Jesus Prayer.
... ~ When you lie down on your bed do not forget to make the sign of the Cross...repeating the Jesus Prayer.
... ~ When you are sleeping and you wake up or get up...make the sign of the Cross...and say the Jesus Prayer.
... ~ When your enter any swelling...say the Jesus Prayer and Amen, and bow to the icon which is in the room.
Russian Old Believer 17th century instructional manual A Son of the Church
Christians ought at all times to preserve the remembrance of God…in order that they may show love to the Lord not only when they go into the place of prayer, but that also when they are working, talking, or eating...
St. Macarius of Egypt (d.391) quoted in The Inner Kingdom
Go to sleep and sleep with the prayer of Jesus. Sts Callistus and Ignatius (14th century) quoted in Writings of the Philokalia
When I arise in the morning, it starts me joyfully upon a new day. When I travel by air, land, or sea, it sings within my breast When I stand upon a platform and face my listeners, it beats encouragement. When I gather my children around me, it murmurs a blessing. And at the end of a weary day, when I lay me down to rest, I give my heart over to Jesus: "(Lord) into thy hands I commend my spirit". I sleep, but my heart as it beats prays on 'Jesus'. Princess Illeana of Romania, princess and Orthodox nun (d.1991), Introduction to the Jesus Prayer
One must force oneself to pray, even if one has no spiritual prayer...In such a case, God, seeing that a man earnestly is striving, pushing himself against the will of his heart (that is, his thoughts), He grants him true prayer.
St. Macarius of Egypt (d.391) quoted by St. Theophan the Recluse
Where Should We Pray the Jesus Prayer?...labor at home to accustom yourself to pray attentively and try to remain in God's presence the rest of the time, as much as possible. Letters of St. Theophan the Recluse (d. 1894)
'Lord Jesus Christ, Son of God, have mercy on me, a sinner.' Let your attention and instruction be centered on this. Walking, eating, standing in church before the beginning of the service, continue with the prayer; on entering and departing keep this prayer on your lips and within your heart. In such a manner, with the invocation of the Name of God you will find peace, you will attain to purity of spirit and body; and the Holy Spirit, the Origin of all good, will make for Himself a dwelling within you and will guide you in all piety and purity.
St. Seraphim of Sarov (d. 1833) quoted in the Jordanville Prayer Book
By orienting yourself to God at all times, your chores at home and responsibilities outside the house will not distract your attention from God, but, on the contrary, will keep you intent on completing all tasks in a God-pleasing manner. All will be performed with the fear of God, and this fear will keep your attention on God unswervingly.
Letters of St. Theophan the Recluse (d. 1894)
It is necessary for everyone, whether eating, drinking, sitting, serving, travelling or doing anything to unceasingly cry Lord Jesus Christ, Son of God, have mercy on me... St. John Chrysostom (d. 407) quoted in the Jordanville Prayer Book
Why Should We Pray the Jesus Prayer?Pray without ceasing.
1 Thessalonians 5:17
...the Lord Jesus Christ instituted prayer in His Name. He promised that such prayer will be particularly effectual.
St. Ignatius Brianchaninov On the Prayer of Jesus
Nor is there salvation in any other, for there is no other name under heaven given among men by which we must be saved.
St. Paul in Acts 4:12
When during prayer your heart is overwhelmed with despondency and melancholy, be sure that these proceed from the Devil, endeavoring by every means to hinder you in your prayer. Be firm, take courage, and by the remembrance of God drive away this deadly feeling.
St John of Kronstadt (d. 1908) On Prayer
Those who have truly resolved to serve God must practice the remembrance of God and unceasing prayer to the Lord Jesus Christ, saying with the mind: Lord Jesus Christ, Son of God, have mercy on me, a sinner.” By this practice, while guarding oneself from distraction and while maintaining peace of conscience, one can draw near to God and be united with Him. Other than by unceasing prayer, according to the words of St Isaac the Syrian, it is impossible to draw near to God. St Seraphim of Sarov (d.1833) quoted in On the Prayer of Jesus
...the saints who for their entire life kept in their heart the Jesus Prayer and the presence of our Lord Jesus Christ. Only they ascend directly to Paradise, for their souls became a paradise while on earth. Where the Lord God is—there is Paradise. Know that if a person trains himself to say the Jesus Prayer continually, when his hour of death comes, Christ Himself will take his soul and it will abide with Him forever.
From the vision of the Nun Tatiana of St. Petersburg in the early 20th century
The name of our Lord Jesus Christ, descending into the depths of the heart, will subdue the serpent holding sway over the pastures of the heart, and will save our soul and bring it to life. Thus, abide constantly with the name of our Lord Jesus Christ, so that the heart swallows the Lord and the Lord the heart, and the two become one. But this work is not done in one or two days; it needs many years and a long time. For great and prolonged labor is needed to cast out the foe so that Christ dwells in us. St John Chrysostom (d. 407) in Writings from the Philokalia
How Should We Pray the Jesus Prayer?"Mental prayer," he continues, "is the result of much vocal prayer, and mental prayer leads to the prayer of the heart. St Ignatius Brianchaninov (d. 1867) quoting Russian hieromonk St. Dorotheus (d. 1865)
When you pray, keep to the rule that it is better to say five words from the depth of your heart than ten thousand words with your tongue only. St John of Kronstadt (d. 1908) On Prayer
For a whole week I stayed along in my hut and recited the Jesus Prayer six thousand times every day, netiehr worrying about anything nor paying attention to the distracting thoughts...I became so accustomed to the Prayer that if for a short while I stopped reciting it I felt as if I were missing somethng.
Anonymous author of The Way of a Pilgrim (19th century Russia); chapter 1
It is used by saying, "Lord Jesus Christ, have mercy on me a sinner, ' and pulling one knot through your fingers. Say it again and move another knot, and so on with each prayer. You may make a prostration, from the waist or to the ground as you desire, at each prayer. Or for small knots, you may do a bow from the waist; large knots, prostrations. The whole rule consists of a fixed number of prayers and prostrations, interspersed with prayers in your own words.
St. Theophan the Recluse (d. 1894) Prayer Rule of Brief Prayers
Let His most sweet name be joined to your breath; and then you will know the profit of silence.*
Gregory the Theologian of Nazianzus (d.391) quoted in Writings from the Philokalia
Take up prayer more readily and continue without interruptions and you will soon achieve your desired goal. Soon a reverent attention to the One God will be established, and with it, inner peace. I say soon, not now, or in a day or two. Months may be required, sometimes, even years. Ask the Lord and He will help. From the letters of St. Theophan the Recluse (d. 1894)
Methods and Steps in the Jesus Prayer
http://www.jesusprayer.us/
