This all-encompassing cosmic unity (Brahman) is not only eternal but also uncreated. It has always existed, of its own accord, and not because of any creator.
There is a conscious divine aspect of Brahman, or Īśvara, who can help us to find realization of the encompassing Brahman.
The world as we know it emerges from Brahman. The many individual beings or ātmans are actually part of that one Brahman, but we, as physical manifestations of those ātmans, are unaware of this fact of our existence.