The following paragraphs are from: 'Meditation and Spiritual Life', Swami Yatiswarananda, Sri Ramakrishna Math, Bangalore, Pg 406 - 407.
The Biblical commandment 'Thou shalt not take the name of the Lord thy God in vain' and Christ's denunciation of 'vain repetition' may be interpreted in several ways. Generally speaking, the stress in Christian religion is more on petitionary prayer than on the repetitition of divine Name, though the Catholics do use the rosary for the repetition of Ave Maria ('Hail Mary'). But when we come to the Greek Orthodox Church, we find great importance given to a kind of repetition of prayer which resembles the Hindu japa. The Greek saints of the Middle Ages perfected a technique of repetition of a simple formula called the Jesus Prayer. In the popular book The Way of a Pilgrim, this technique is described as follows:
The continuous interior Prayer of Jesus is a constant uninterrupted calling upon the divine Name of Jesus with the lips, in the spirit, in the heart; while forming a mental picture of His constant presence and imploring His grace, during every occupation, at all times, in all places, even during sleep. The appeal is couched in these terms, 'Lord Jesus Christ, have mercy on me'.
In Islam the Sufi mystics have for centuries employed the repetition of Allah or Ali as a means of getting spiritual illumination. In the school of Al-Ghazali, the great Muslim theologian of the twelfth century, the instruction given to the initiates is as follows:
Let the seeker...sit alone in some corner, let him see to it that nothing save God--the Most High--enters his mind. Then as he sits alone in solitutude, let him not cease saying continuously with his tongue 'Allah, Allah', keeping his thought on it. At last he will reach a state wherer the motion of his tongue will cease, and it will seem as though the word flowed from it. Let him persevere in this, until all trace of motion is removed from his tongue and he finds his heart persevering in the thought. Let him still persevere until the form of the word--its letters and shape--is removed from his heart, and there remains the idea alone, as though clinging to his heart, inseparable from it. Nothing now remains but to await what God will open to him. If he follows the above course, he may be sure that the light of the Real will shine in his heart.