"The Upanishads teach monism, that all is God or the Absolute, Sarvam Khalvidam Brahma, “Everything is Brahman”. But they do not do this in simply an abstract manner. That One Being is present in all of us as our own deeper and immortal soul and Self, the Atman, Aham Brahmasmi, “I am Brahman” or the Absolute.
In this regard, the Upanishads probably first clearly set forth in human history a way of Self-Knowledge taking us to the Absolute. Yet theism is also present in many places in the Upanishads, a recognition of One God or Isvara as the creator, preserver and destroyer of the universe and the ability to unite with Him (or Her) through meditation.
The Upanishads also say Ishavasyam Idam Sarvam, “All this universe is pervaded by the Lord"