In Manduka Upanishad Brahman and Atman are defined as same.+

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:~

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

Translation:

Sarvam(सर्वम्)- Whole/All/Everything; hi(हि)- Really/Just/Surely/Indeed; etad(एतद्)- This here/This; brahm(ब्रह्म)- Brahm/Brahman; ayam(अयम्)- This/Here; aatmaa(आत्मा)- Atma/Atman; sah(सः)- He; ayam(अयम्)- This/Here; chatus(चतुस्)- Four/Quadruple; paat(पात्)- Step/Foot/Quarter

Fragmented Verse:~ सर्वम् हि एतद् ब्रह्म अयम् आत्मा ब्रह्म सः अयम् आत्मा चतुस पात् / sarvam hi etad brahm ayama aatmaa brahm sah ayam aatmaa chatus paat

Simple Meaning:~

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.

Imagine a person who is blind from birth and has not seen anything. Is it possible for us to explain to him the meaning of the color red? Is any amount of thinking or reasoning on his part ever going to make him understand the sensation of the color red? Similarly, the idea of Brahman cannot be explained or understood through material reasoning or any form of human communication. Brahman is like the color red; those who can sense it cannot explain or argue with those who have never sensed it.

Bhagavad Gita:~ brahmano hi pratisthaham-  Brahman is considered the all-pervading consciousness which is the basis of all the animate and inanimate entities and material. (14.27)

In Advaita Vedanta: - Brahman is without attributes and strictly impersonal. It can be best described as infinite Being, infinite Consciousness, and infinite Bliss. 

It is pure knowledge itself, similar to a source of infinite radiance. Since the Advaitins regard Brahman to be the Ultimate Truth, so in comparison to Brahman, every other thing, including the material world, its distinctness, the individuality of the living creatures, and even Ishvara (the Supreme Lord) itself are all untrue.

Brahman is the effulgent cause of everything that exists and can possibly exist. Since it is beyond human comprehension, it is without any attributes, for assigning attributes to it would be distorting the true nature of Brahman. Advaitins believe in the existence of both Saguna Brahman and Nirguna Brahman; however, they consider Nirguna Brahman to be the absolute supreme truth.

Chandogya Upanishad:- One who meditates upon and realizes the self discovers that everything in the cosmos-- energy and space, fire and water, name and form, birth and death, mind and will, word and deed, mantram and meditation--all come from the Self.

So, it clearly says the one who meditates upon the self [consciousness] discovers that everything in the cosmos-- energy and space, fire and water, name and form, birth and death, mind and will, word and deed, mantram and meditation--all come from the Self. Therefore, there is a need to know the fact that, the true self is not physical but the soul to realize the fact that: the cosmos-- energy and space, fire and water, name and form, birth and death, mind and will, word and deed, mantram and meditation--all come from the Self, which is in the form of consciousness.

Atman is Brahman. Brahman /God in truth/the Absolute is alone real; this waking is unreal, and the three states are non-different from Brahman/God in truth

Whatever is, is Brahman/God in truth. Brahman/God in truth itself is absolutely homogeneous. All difference and plurality are illusory." Brahman/God in truth is not a person, as the Absolute is not this. But if one wants to call it God/Paramataman, then fine. But it is not a person.

Personifying  God can make it difficult to understand and realize God in truth. :~Santthosh Kumaar