It looks like you are asking about the thesis or main idea for The Way to Rainy Mountain.
Essentially, the novel is made up of a collection of stories about Kiowa history, Kiowa culture, and Momaday's family history. If I were to sum up the main idea, I would argue that memory (assisted by verbal tradition) is an important preserver of culture, especially when there are few recorded events to rely on. Both the verbal tradition and recorded history contribute immensely to how history is remembered.
In the book, Momaday traces the history of his people, the Kiowas. He tells of their migration from the headwaters of the Yellowstone River to the south of the Wichita Mountains. The Kiowas were an adaptive people; they allied with the Comanches and the Crows, acquired a large number of horses, and adopted the sun religion of the Plains Indians.
Horses became extremely important to the Kiowas after the buffalo was decimated. In fact, the Kiowas began naming their religious rites as the "horse-eating sun dance" or Tsen-pia Kado. Momaday holds the summer of 1879 as the time the buffalo disappeared completely from Kiowa life.
This mention of the loss of the buffalo is pivotal to the story: it underlines the resilience of the Kiowa people, who fought desperately to retain their adopted way of life. Even after the last Kiowa warriors surrendered to the United States cavalry, members of the tribe relied on their adaptive natures to navigate an alien culture. Momaday's grandmother chose to convert to Christianity. Yet, she retained a fierce pride in her Kiowa heritage.
Momaday also writes about how there were only ten fighters in the Kiowa warrior society. These warriors were called the Ka-itsenko (Real Dogs), and their battle prowess was revered by all. Momaday relates that there were always dogs lingering around his grandmother's house when she was alive.
Most importantly, Momaday's knowledge of Kiowa history has been reinforced by testimonies from older members of the tribe such as Ko-Sahn, an acquaintance of Momaday's grandmother. At the time of her interaction with the author, Ko-sahn was one hundred years old. Yet, her memories of the Sun dances were vivid, and she related scenes from those events with both pride and wistful longing.
In the book, Momaday's retelling of Kiowa history is supported by both recorded history and verbal tradition. He makes the point that much is gained when we rely on both to help us envision the past and to preserve important traditions for future generations.