In his great book ‘The dream in Native American and other primitive cultures' (1935, 2003, Dover books, Mineola, NY) Jackson Steward Lincoln built upon Sir Edward Tylor's suggestions that most religious ideas of people in small-scale traditional societies come from dreams. Both Tylor and Steward Lincoln presented many examples of religious ideas emerging from dreams and then informing the ritual ceremonies and religious beliefs of the people. Essentially dreams functioned to identify the sacred and profane realms and were considered a portal to the supernatural.
From the days of first contact European observers of Native American societies have consistently remarked upon the extreme significance given to dreams by native American peoples. In many cases the dream was accorded an equal or greater reality than waking. Dreams were a source of power while waking reality was the arena of struggle and conflict. Tylor and Steward Lincoln noted that belief in spirit beings was justified by pointing to the dream world.
In the dream there are characters invested with supernatural powers and they address the dreamer, they communicate with him, they can harm the dreamer and they can bestow gifts upon the dreamer. Thus arose the religious idea of the soul or spirit being.
The idea that there was life after death also arose from dreams, because dreamers could see people who had died and yet lived again in their dreams. These ‘dead people' would speak about their ‘life' in the spirit world to the dreamer and warn the dreamer about things in his everyday waking world. Thus arose the religious idea of the immortality of the soul.
Because deceased ancestors appeared so frequently in dreams of the people ancestor reverence and worship emerged as one of the earliest of mankind's religious rituals. Similarly, animals important to the tribe or clan also very often appeared in dreams and interacted with the dreamer and so arose various forms of ritual animal worship.
In addition the religious idea of the transmigration of souls probably arose from dreams as the dreamer regularly saw people and spirit beings in dreams turn into animals and vice versa...or one person turning into another and so on. Steward Lincoln suggests furthermore that the elements of totemism originated in dreams as well.
Ritual proscriptions concerning actions may have also been influenced by the dream as many peoples created rules that followed the form "IF you dream of X, THEN perform action Y". For example, if an Omaha boy on a vision quest dreamt of an animal with supernatural power he would have to upon waking find the animal, kill it (sacrifice it), then take a part of it as a talisman. The talisman would then be sung and prayed over with songs and prayers derived in part from dreams and ultimately utilized in religious rituals throughout the individual's life.
There is now a great debate within the cognitive and psychological sciences on the origin of religious ideas-yet I have yet to see any specialist in this area consider dreams as a source of religious ideas. Kelly Bulkeley's excellent works are, unfortunately, the exception that prove the rule. I predict that once dreams begin to be rigorously examined as creative sources of cultural artifacts including religious ideas and rituals then the world of dream scholarship will really begin to take off. As it is the study of dreams has barely begun in earnest.