Catechism of the Catholic Church

2667
This simple invocation of faith developed in the tradition of prayer under many forms in East and West. The most usual formulation, transmitted by the spiritual writers of the Sinai, Syria, and Mt. Athos, is the invocation, "Lord Jesus Christ, Son of God, have mercy on us sinners." It combines the Christological hymn of Philippians 2:6-11 with the cry of the publican and the blind men begging for light. Mk 10:46-52

2668
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 is not lost by heaping up empty phrases, but holds fast to the word and "brings forth fruit with patience." Lk 8:15 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.