Bede Griffiths, and the Shantivanam Ashram in India. It has sustained me through difficult times. If I feel down, or feel out of touch with God, or any hopelessness, I pray the mantra, and I feel assured of God's Presence, even if the pain remains. It is part of my daily spiritual practice, and has been for awhile.
The sacred syllable "Om" is the name of God, and the vibration of the supreme. It is the Word of God.
For the Christian practitioner, the focus of the Mantra is the Word, Christ.
The New Testament represents Christ as the Word made Flesh:
"In the beginning was the Word, the Word was with God, and the Word was God." - John 1.1.
"For in him, the complete being of God, by God's own choice, came to dwell..." - Ephesians 1.19, NEB.
"For it is in Christ that the complete Being of Godhead dwells embodied." - Ephesians 2.9, NEB.
It is the Cosmic Christ that we worship, who "fills the universe in all its parts" (Ephesians 1.23).
The Mantra focuses not only on the Divinity of Christ, but his name, for "The name of the LORD is a strong tower..." - Proverbs 18.10.
It is by union with Christ that we realize our own divinity, for "as many as received him, he gave them power to be made the sons of God, to them that believe in his name," - John 1.12, Douay-Rheims, and "For both he that sanctifieth, and they who are sanctified, are all of one. For which cause he is not ashamed to call them brethren..." - Hebrews 2.11, Douay-Rheims.
George Harrison famously said that repeating the mantra was like having "God dance on your tongue."
Indeed, Jesus said "If any one love me, he will keep my word, and my Father will love him, and we will come to him, and will make our abode with him."- John 14.23, Douay-Rheims.
To practice the Mantra is to practice the Presence of God. For the Christian, this means honoring the Name of Christ. For it is in Christ "that you have been brought to completion." - Colossians 2.10, NEB.