Manduka Upanishad Brahman and Atman are defined as same.+

In Manduka Upanishad Brahman and Atman are defined as same:~ सर्वं ह्येतद् ब्रह्मायमात्मा ब्रह्म सोयमात्मा चतुष्पात् / sarvam hyetad brahmaayamaatmaa brahm soyamaatmaa chatushpaat –

Manduka Upanishad, verse-2:~All indeed is this Brahman; This Atman is Brahman; God, this Atman has four steps/quarters.

While Brahman lies behind the sum total of the objective universe, some human minds boggle at any attempt to explain it with only the tools provided by reason. Brahman is beyond the senses, beyond the mind, beyond intelligence, beyond imagination. 

Indeed, the highest idea is that Brahman is beyond both existence and non-existence, transcending and including time, causation, and space, and thus can never be known in the same material sense as one traditionally 'understands' a given concept or object. 

Rig Veda: ~ 'Prajnanam Brahma'- Consciousness is the ultimate reality or Brahman or God in truth.

Do not accept any other God other than the Soul. The Soul is God in truth,  Nothing is real but the Soul, which is present in the form of consciousness. Nothing matters but realize  God in truth. God in truth is everywhere and in everything. Let these words be inscribed in your subconscious.

God in truth is hidden by the illusory universe. God in truth alone is real and eternal and all else is an illusion.

Brahman is merely a word to indicate the ultimate truth or God in truth.  The ultimate truth itself is God in truth. 

Remember:~

When the Soul, the 'Self' remains in its own awareness: ~

What is this body?

What is the ego?

What is this world?

What is the 'I'?

What is duality?

What is existence, nonexistence, unity, or duality?

What needs us there to say more?

Noting from the Soul because whatever seems to emanate from it, is non-different from itself. There is no second thing other than the Soul, which is present in the form of consciousness.

One should not mistake Self, which is the Soul for the ‘I’.

Those who assert the world is a reality, are still in the elementary stages of the preliminary analysis. The world is a reality within the waking experience, but the waking experience is merely an illusion. The same way the dream world was a reality within the dream experience. The dream became unreal when the waking took place. The same way the waking becomes unreal when Advaitic wisdom dawns. : ~ Santthosh Kumaar