Aum, the breath, is related to the Word as the Holy Spirit is to Christ, as Atman is to the I.

"The word ‘Aum’ is the breath. The breath is related to the Word as the Holy Spirit is to Christ, as Atman is to the I."  — Rudolf Steiner

Source: http://wn.rsarchive.org/Lectures/GA/GA0093a/19050927p01.html