The fundamental idea of the Upanishads is the unity of existence – there is only ‘one reality without any second’. This is the simple teaching of Upanishad. All other concepts and theories are built around this fundamental teaching.
The Upanishads are replete with ideas like, ‘All this [phenomenal universe] is indeed Brahman’, ‘Here there is no multiplicity’, ‘All this [universe] is pervaded by the Supreme Truth’,11 and so on.
And also the Upanishads make no distinction between the individual self and the universal Self. The mahavakyas, great sentences, like ‘I am Brahman’ and ‘You are That’, speak eloquently about the identity of the individual and the universal Self.
Physically this universe is one: there is no difference between the sun and you.
The ultimate goal of Upanishads is identification of jiva with Brahman, the universal Reality or God.