Religions answer questions like that based on universal mystical experiences. I think the more you examine comparative mysticism, comparative mythology, and comparative religion, the more apt you are to conclude that all religions are esoterically related even when separated by time and space.
That's an unacceptable gloss that seems to be based on a few facts, but not on a thorough knowledge of the subject. For instance, there is no hard evidence that the most popular Christinan notions of the self are at all based on mystical experiences. To believe that is the case strikes me as wishful thinking. As for the Dakota, they seem to have had a radically different concept of the self before contact with Europeans. After contact, some syncretism seems to have taken place. And the Zen Buddhist notion of the self is so different from the Christian that most Christianns and former Christians simply do not accept it at face value, but attempt to interpret it and re-interpret it, and re-interpret it again -- until they have something similar to the Christian view.
When I began studying comparative mysticism over 35 years ago, I too was more or less of the opinion that (1) all religions were based on mysticism, and (2) that religions had a fundamental, esoteric unity. But that, based on what I've learned in the past 35 years, was a premature conclusion based on too little information.