Mr. ... makes reference to Mr. Taherzadeh's "The Revelation of Bahá'u'lláh", vol. IV, (Oxford: George Ronald, 1987), pp. 133-134, where mention is made of the fact that the Most Great Spirit "animated and sustained" Bahá'u'lláh. In light of this section, he enquires about the difference between the Holy Spirit and the Most Great Spirit.
The Research Department has, to date, not been able to locate a comprehensive definition of the term "Most Great Spirit" in the Writings or the letters of Shoghi Effendi. The discussion in Mr. Taherzadeh's book appears to be based, on part, on an extract from the Súriy-i-Haykal which states:
The Holy Spirit Itself hath been generated through the agency of a single letter revealed by this Most Great Spirit, if ye be of them that comprehend. (As translated and cited by Shoghi Effendi in "The World Order of Bahá'u'lláh: Selected Letters", p. 109)
Shoghi Effendi has provided an interpretation of this extract in a letter dated 23 July 1936 written on his behalf to an individual believer in response to a series of questions about the relationship between the Holy Spirit and Bahá'u'lláh and His relationship to the other Manifestations of God. The letter states:
As to your question concerning the Holy Spirit and its relation to Bahá'u'lláh: the Holy Spirit may be well compared to the rays of the sun, and Bahá'u'lláh to a perfect mirror reflecting these rays which radiate from the sun. Briefly stated the comparison is this: God is the sun; the Holy Spirit is the rays of the sun; and Bahá'u'lláh is the mirror reflecting the rays of the sun. In the passage you have quoted from the "Súriy-i-Haykal" Bahá'u'lláh refers to His station of identity with God, to His reality which is Divine. In this passage it is really God speaking through Bahá'u'lláh. Bahá'u'lláh is not the intermediary between God and the other Manifestations, although these are under His shadow, for the simple reason that the Messengers of God are all inherently one; it is their Message that differs. Bahá'u'lláh appearing at a time when the world has attained maturity, His message must necessarily surpass the message of all previous prophets. Not only so, but His message is potentially greater than any message which later prophets within His own cycle may reveal. This is because the stage of maturity is the most momentous stage in the evolution of mankind...
In "God Passes By" (Wilmette: Bahá'í Publishing Trust, 1987), p. 101, Shoghi Effendi describes the coming of Revelation to Bahá'u'lláh in the Siyah-Chal and makes the following statement about how the "Most Great Spirit" was manifested symbolically in earlier Dispensations. He wrote:
...at so critical an hour and under such appalling circumstances the "Most Great Spirit", as designated by Himself, and symbolized in the Zoroastrian, the Mosaic, the Christian, and Muhammadan Dispensations by the Sacred Fire, the Burning Bush, the Dove and the Angel Gabriel respectively, descended upon, and revealed itself, personated by a "Maiden" to the agonized soul of Bahá'u'lláh.
From the foregoing, it appears that the term the "Most Great Spirit" is used to convey both the kindling of Revelation in the Manifestations of God and God speaking through His Manifestations.
(From a memorandum dated 30 December 1991 prepared by the Research Department of the Universal House of Justice and included in a letter dated December 30, 1992, written on behalf of the Universal House of Justice to an individual believer)