Wilber draws a crucial distinction between the 'average mode' of religious consciousness and the 'leading edge.' Leading-edge pioneers break through into new states of consciousness and then leave descriptions and instructions whereby others can follow them.
Wilber's division of spiritual states into four broad classes of gross, subtle, causal, and nondual is helpful. He suggests that just as these tend to emerge sequentially in today's contemplatives, so too did they emerge in history, and a survey of historical religious texts supports him.
Humankind's first gross and subtle spiritual experiences are long lost in the dawn of prehistory. Reliable signs of the causal appeared in the first few centuries before the Common Era and are associated with, for example, the Upanishads, the Buddha, early Taoists, and later with Jesus. Such was the extraordinary impact of this breakthrough and the sages who made it, that this era is known as the Axial age. Signs of the nondual appear a few centuries into the Common Era and are associated with, for example, the appearance of tantra and with names such as Plotinus in Rome, Bodhidharma in China, and Padmasambhava in Tibet.
... the evolution of religious consciousness is intimately tied to the evolution of the technology of transcendence. There has been an enormous development of transformational techniques ... early sages refined shamanic technology, added their own yogic, contemplative, and tantric techniques, and thereby created technologies that unveiled causal and nondual realizations. Subsequently, religions, philosophies, and psychologies arose to express and analyze these realizations, and their cumulative impact on human culture and consciousness is inestimable."
Roger Walsh. “The World of Shamanism. New Views of an Ancient Tradition.” Llewellyn Publications, Woodbury, Minnesota, 2007, p259-60.