If one firmly believes that consciousness is generated by the brain, the following will be of no use, or may even generate feelings of scorn. But then, such belief by scientists, though ubiquitously assumed, utterly fails scientific verification, and is one of the biggest enigmas of modern understanding, yet for at least 4000 years, seers have known (undoubtedly with the help of sacraments) the answer. They even wrote it down to be available to all.
And the answer to this mystery is... the "oneness" is Consciousness alone. All the
apparent Universe of objects exist in the infinite, eternal One which is only Consciousness.
In ignorance of this truth, we take objects appearing within consciousness including our individuality, bodies and the Universe to be "real."
In a dream, all objects including the "self" character in the dream, have no existence outside the consciousness of the dreamer.
Vedanta describes ultimate reality as "Being Consciousness Bliss (Sat Chit Ananda)."
What part do Galactic superclusters and atoms, and Life have in Being Consciousness Bliss?
What else would eternal, pure, infinite and undifferentiated Consciousness do, except dream up a differentiated experience full of intrigue and play?
"There is no getting behind Consciousness." - Max Planck ("cherry picked," but illustrative

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As to why this is not "taught" by sages, as other have said, it is indeed, if one can consider the "set & setting" of times and people the teachings were recorded by (and then sadly often altered) through history.
One has to be "ripe" to hear this. One has to be ready to decondition one's beliefs and be ready to give up the idea that the reality we experience is veridical... Dr. Donald Hoffman, Dr. Andrew Gallimore, and a few DMT flashes can help with this
