On page 48 of Metaphysical Meditations, Paramahansa Yogananda writes:
EXLANATION OF “AUM” AND “CHRIST CONSCIOUSNESS”
In Autobiography of a Yogi Paramahansa Yogananda says: ” ‘The Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said to you” (John 14:26). These Biblical words refer to the threefold, nature of God as Father, Son, Holy Ghost (Sat, Tat, Aum in the Hindu scriptures).
“God the Father is the Absolute, Unmanifested, existing beyond vibratory creation. God the Son is the Christ Consciousness existing within vibratory creation; this Christ Consciousness is the ‘only begotten’ or sole reflection of the Uncreated Infinite.
“The outward manifestation of the omnipresent Christ Consciousness, it’s ‘witness’ (Rev. 3:14), is Aum (Om), the Word or Holy Ghost: invisible divine power, the only doer, the sole causative and activating force that upholds all creation through vibration. Aum the blissful Comforter is heard in meditation and reveals to the devotee the ultimate Truth, bringing ‘all things to your remembrance.’ “