From " Mejda: The Family and the Early Life of Paramahansa Yogananda"

 The three aspects of God — Creator, Preserver, Destroyer — and all His qualities are contained in the Aum vibration. To concentrate on Aum is the surest way to realization of Brahman. The sacred scriptures —the Vedas, Upanishads, Gita, Bhagavata — all declare that God may be known through Aum.

Aum is the best symbol for those who want to worship a personal God. It is a better symbol than images, such as those of Kali, Krishna, and others; because in worshipping God as an image, the mind limits the Infinite to that form. It becomes difficult for the sadhaka to think of God beyond that ishta, when in fact God is Spirit.

While meditating on Aum, the mind expands with the Omnipresence of that vibration, and the Spirit-nature of Brahman inherent therein is realized as Sat-chit-ananda — ever-existing, ever- conscious, ever-new Bliss.