I must be the anomaly, I love his writing style, he writes like Queen Elizabeth I court talked, never say anything with one word when 15-20 will do. I get great mental pictures from his words.
QUOTING
An example in point, as ancient as it is eloquent, is the idea of the trinity and its emblem, the triangle. What the human thought of God is depends on what power of the mind or aspect of life man uses as a lens through which to look into the mystery of things. Conceived of as the will of the world, God is one, and we have the monotheism of Moses. Seen through instinct and the kaleidoscope of the senses, God is multiple, and the result is polytheism and its gods without number. For the reason, God is a dualism made up of matter and mind, as in the faith of Zoroaster and many other cults. But when the social life of man becomes the prism of faith, God is a trinity of Father, Mother, Child. Almost as old as human thought, we find the idea of the trinity and its triangle emblem everywhere—Siva, Vishnu, and Brahma in India corresponding to Osiris, Isis, and Horus in Egypt. No doubt this idea underlay the old pyramid emblem, at each corner of which stood one of the gods. No missionary carried this profound truth over the earth. It grew out of a natural and universal human experience, and is explained by the fact of the unity of the human mind and its vision of God through the family.
UNQUOTE
Boil that down to: g-d is universal and comes from Man's experience as he interacts with others and with nature. Interestingly most religions do indeed have a triune godhead. From Judaism/Christianity through you name it, there is almost always three aspects to Deity. Therefore the universality of the triangle shape.