I have been reading in Bulgakov's contraversial works on Divine Wisdom, and the following quotation stood out for me as expressing especially well part of his theological position concerning the Blessed Mother Mary and the person and work of the Holy Ghost:
The veneration of our Lady in Orthodoxy is such that those outside may well ask the question, Is not this frankly exaggerated? Does it not introduce into Christian doctrine the figure of a goddess? Such a misapprehension may be dissipated by the simple consideration that our Lady, however exalted may be the honour paid her, is not divine, is not even theandric. Her human nature and personality subsist in spite of her complete deification. Though she, upon whom the Holy Ghost reposes, is therefore spirit-beraing, yet for all that she remains a woman, however fully deified. The Holy Ghost is not personally incarnate like the Son. In conformity with his personal nature he blesses, sanctifies, penetrates, and vivifies, and that is all. And yet his fullest and loftiest manifestation is nevertheless effected in the spirit-bearing "blessed" Virgin Mary. She is, in personal form, the human likeness of the Holy Ghost. Through her, with her human form become entirely transparent to the Holy Ghost, we have a manifestation and, as it were, a personal revelation of him.
The person of the Holy Ghost remains hidden from us even in his descent at Pentecost, which conferred immediately only the gifts of the Spirit. But there is a human person to whom it is given to manifest the Holy Ghost himself, and that is the most holy Virgin, Mary, the heart of the Church. And yet this manifestation of the Holy Ghost—let us emphasise the fact that it is precisely a manifestation, not an incarnation—remains for us in this life beyond our understanding.
Father Sergei Bulgakov
The Wisdom of God: A Brief Summary of Sophiology
Excerpted from 'The Veneration of Our Lady'
Williams and Norgate, Ltd.