So it has reached us and so it is observed by us and will pass in the same form to those who come after us. As regards the words: ‘Have mercy upon me’ added to the salvation-working words of the prayer ‘Lord Jesus Christ, Son of God’, it was added by the holy fathers chiefly for those who are still infants in the work of virtue, the beginners and the the imperfect. For the advanced and the perfect in Christ are content with any one of those forms: ‘Lord Jesus’; ‘Jesus Christ’; ‘Christ Son of God’; or even with one word ‘Jesus’, which they kiss and embrace as the complete doing of the prayer, sufficient to fill them with ineffable bliss and joy exceeding all mind, all vision and all hearing.
Ss. Callistus and Ignatius, Directions to Hesychasts, 50.