The birth described is the coming forth of the divine self within a person, conceived not by the flesh but by the descent of the highest spirit upon the receptive soul. The Holy Ghost is the pure breath and pneuma, the animating spirit that comes upon the pure feminine principle, Mary, whose name carries the sea and the bitter waters, the deep receptive medium in which the higher life takes form. To be overshadowed by the power of the Highest is to be covered and enveloped by the source-energy above the ordinary mind, so that conception happens in shadow, in the hidden inner chamber where the outer eye cannot follow. The holy thing born of this union is called the Son of God because it is generated from spirit alone, the Christ-consciousness rising in the individual.
The overshadowing is the key movement, the higher covering the lower until something new takes shape between them. What is born is not the child of the body but the awakened divine nature in a person, brought forth in the still inner room where spirit meets the soul made ready to receive it.
Holy Ghost: the pure animating breath and pneuma, the spirit-energy that comes upon the receptive soul
Highest: the source-consciousness above the ordinary mind, whose power envelops and covers the soul
Son of God: the Christ-consciousness born of spirit alone within a person, the awakened divine self