The most common form of unceasing prayer in the Orthodox tradition is the Jesus Prayer. The Jesus Prayer is the form of invocation used by those practicing prayer, also called the “prayer of the heart.” The words of the prayer most usually said are “Lord Jesus Christ, Son of God, have mercy on me a sinner.” The choice of this particular verse has a theological and spiritual meaning.
First of all, it is centered on the name of Jesus because this is the name of Him whom “God has highly exalted,” the name given to the Lord by God Himself (Lk 1.31), the “name which is above every name” (Phil 2.9–10, cf. Eph 1.21).
. . . for there is no other name given among men by which we must be saved (Acts 4.12).
All prayer for Christians must be performed in the name of Jesus: “if you ask anything in My name, I will do it” (Jn 14.13–14).
The fact that the prayer is addressed to Jesus as Lord and Christ and Son of God is because this is the center of the entire faith revealed by God in the Spirit.
He said to them, “But who do you say that I am?”
Simon Peter replied, “You are the Christ, the Son of the Living God.”
And Jesus answered, “Blessed are you . . . for flesh and blood has not revealed this to you, but my Father who is in heaven . . . and on this rock I will build my Church . . .” (Mt 16.16–18).
That Jesus is the Christ, and that the Christ is Lord is the essence of the Christian faith and the foundation of the Christian church. To believe and proclaim this is granted by the Holy Spirit.
. . . no one can say “Jesus is Lord” except by the Holy Spirit (1 Cor 12.3).
. . . every tongue should confess that Jesus Christ is Lord to the glory of God the Father (Phil 2.11).
In calling Jesus the Son of God is to acknowledge God as His Father. To do this is, at the same time, to have God as one’s own Father, and this too is granted by the indwelling Spirit.
And when the time had fully come, God sent forth His Son, born of a woman, born under the law, to redeem those who were under the law, so that we might receive adoption as sons. And because you are sons, God has sent the Spirit of His Son into our hearts, crying “Abba! Father!” (Gal 4.4–6).
When we cry “Abba! Father!” it is the Spirit Himself bearing witness with our spirit that we are children of God . . . (Rom 8.15–16).
Thus, to pray “Lord Jesus Christ, Son of God” is already to be a child of God, and already to be certain that the Holy Spirit is in you. In this way, the Jesus Prayer brings the Spirit of God into the heart of man.
“Have mercy on me a sinner” is the publican’s prayer. When uttered with humble conviction it brings divine justification (cf. Lk 18.9–14). Generally speaking, divine mercy is what man needs most of all. It is for this reason that the numberless repetition of the request for the Lord’s mercy is found everywhere in the prayers of the Church.
And finally, all men are sinners. To know this is a fact, and to confess it with faith is to be justified and forgiven by God (cf. Rom 3.10–12, Ps 14.1–3).
The Jesus Prayer basically is used in three different ways. First as the verse used for the “prayer of the heart” in silence in the hesychast method of prayer. Second as the continual mental and unceasing prayer of the faithful outside the hesychast tradition. And third as the brief ejaculatory prayer used to ward off temptations. Of course, in the actual life of a person these three uses of the prayer are often interrelated and combined.