Vedānta says, सर्वं खल्विदं ब्रह्म sarvaṁ khalvidaṁ brahma – All of this is Brahman – Chāndogya Upaniṣad 3.14.1.
All this multiplicity, diversity is but Brahman only.
Brahman is one, without a second. एकमेवाद्वितीयम् ब्रह्म ekamevādvitīyam brahma – Chāndogya Upaniṣad 6.2.1
There is no second thing here. Śaṅkara says that all this multiplicity is imagined only in Brahman. सर्वम् हि नानात्वम् ब्रह्मनि कल्पितमेव sarvam hi nānātvam brahmani kalpitameva
Brahman means Absolute Reality.
It is described as Sat-Chit-Ananda: Pure Existence, Consciousness, and Bliss.
It is called Pure Existence because it is beyond time, space and causality. Normally, the existence of anything depends upon time, space and causality. Whereas Brahman as pure existence lends existence to time, space, and causality! That is why it is called pure existence—meaning, it does not depend upon time, space, and causality. They depend upon it!
It is Pure Consciousness because it is bereft of objects, yet all objects depend upon it. Everything internal (thoughts, feelings, desires etc.) and external (the perceived world, universe) are objects to it. Their existence depends upon Consciousness. Put differently, everything other than consciousness has dependent existence only. They cannot exist apart from it.
It is Pure Bliss because it is uncaused. All experiential happiness is caused—it depends upon circumstances, person (s), or objects. It has a beginning. Because, it has beginning, it has an end. It is limited by time and space. But Brahman is Pure Bliss because it is free from the duality of the other. It is Infinite Bliss. This bliss is not caused by anything else because there is no second thing to Brahman. This Infinite Bliss is its very nature.
Then the Upanisaḍ makes a most unusual statement. This Brahman which is Pure Existence, Consciousness, and Bliss is YOU! You are THAT! It is not that you will become THAT after you have meditated for several years or has done this or that spiritual practice or after death; You are THAT right here, right now. Period!
The whole beauty of this is that it is not an intellectual claim that should be taken on faith.
A teacher who has realized the truth of Brahman can show it in the direct experience of the student. It can be established in the common day-to-day experience of everyone.
This Brahman is available to us all in our own experience as the consciousness-presence. The felt presence of our being. As the Witnessing Consciousness of our thoughts, feelings, desires, including our waking, dream and deep sleep states.
That we exist and that we are aware that we exist–these are undoubtable facts that can never ever be negated. This is our true Self, which is never not one with Brahman.
This is the declaration उद्धघोष of Vedānta.
We conclude with a short Song of the Self:
Your true Self is beyond time and space:
It is always right here, right now.
Any step toward the Self is actually a step away from it.
This journey is one of re-discovery and re-cognition.
No journey actually, but a home-coming.
Welcome yourself Home.