In Upanishads the God has been described as:~ ekam evaditiyam- God is only one without a second.+

Hindus are idol worshipers of a large number of Gods and Goddesses whereas in the Vedas the God has been described as:~

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. 

Rig-Veda 1-164-46 and Y.V 32-1 clearly mention that God is “One”.

Rig Veda: ~ The Atman is the cause; Atman is the support of all that exists in this universe. May ye never turn away from the Atman, the Self. May ye never accept another God in place of the Atman nor worship other than the Atman?" (10:48, 5)

Yajurveda ~ chapter- 32:~    God is  Supreme Spirit has no ‘Pratima’ (idol) or material shape. God cannot be seen directly by anyone. God pervades all beings and all directions. Thus,   Idolatry does not find any support from the Vedas.

Thus,  it refers to formless and attributeless God, which is the Atman (Soul), the innermost Self within the false experience. Thus, it indicates clearly all the Gods with form and attributes are mere imagination based on the false self.  Thus Atman or Soul, the  Self is God in truth.

The Vedas do not talk about idol worship. In fact, till about 2000 years ago followers of Vedism never worshiped idols. Idol worship was started by the followers of Buddhism and Jains.  There is logic to idol worship. Vedas speak of one God that is the supreme Self i.e. Atman or Soul but Hinduism indulges in worshiping 60 million Gods.

In  Upanishads, God has been described as:~

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.

Brihadaranyaka Upanishad: ~ Brahman (God) is present in the form of the Athma, and it is indeed Athma itself.

Chandogya Upanishad Chapter: ~ekam evaditiyam- God is only one without a second. (6- Section- 2- Verse- 1)

Swethaswethara Upanishad: ~ Na casya kasuj janita na cadhipah, which means of him of Almighty God, there are no parents they have got no lord. Almighty God has no true father, he has no true mother, he has no true superior. (Chapter-6- Verse -9)

Swethaswethara Upanishad: ~ “Na Tasya Pratima Asti- of that God there is no Pratima, there is no likeness, there is no image, there is no picture, there is no photograph, there is no sculpture, there is no statue. (Chapter -4- Verse- 19)

Swethaswethara Upanishad: ~ “No one can see the Almighty God. (Chapter -4, Verse -20)

Brihadaranyaka Upanishad:~  The Self is indeed Brahman (God, but through ignorance, people identify it with intellect, mind, senses, passions, and the elements of earth, water, air, space, and fire. This is why the Self is said to consist of this and that and appears to be everything. 

In Bhagavad Gita, God has been described as:~

Lord Krishna says Ch. V:~ “Those who know the Self in truth.". The last two words (tattvataha) are usually ignored by pundits, but they make all the difference between the ordinary concept of God and the truth about God.

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

Bhagavad Gita Chapter:~  All those whose intelligence has been stolen by material desires, they worship many Gods. (7- Verse -20)

According to Sage Sankara says: ~ Ataman is Brahman (God). Thus, the Soul the innermost self is God. Therefore,    all the Gods with form and attributes are mere imaginations based on the false self.  Thus, there are adulteration and add-ons in the past, which has to be bifurcated if one wants pure Vedic essence.  

That is why Swami Vivekananda:~ The masses in India cry to sixty million Gods and still die like dogs. Where are these Gods?

 Knowing this, stand up and fight! Not one step back that is the idea. ... Fight it out, whatever comes. Let the stars move from the sphere! Let the whole world stand against us! Death means only a change of garment. What of it? Thus fight! You gain nothing by becoming cowards. ... Taking a step backward, you do not avoid any misfortune. You have cried to all the Gods in the world. Has misery ceased? The masses in India cry to sixty million Gods and still die like dogs. Where are these Gods? ... The Gods come to help you when you have succeeded. So what is the use? Die game. ... This bending the knee to superstitions, this selling yourself to your own mind does not befit you, my Soul. You are infinite, deathless, birthless. Because you are the infinite spirit, it does not befit you to be a slave. ... Arise! Awake! Stand up and fight! Die if you must. There is none to help you. You are the entire world. Who can help you? - Swami Vivekananda  (Delivered In San Francisco, on May 28, 1900) -The Complete Works of Swami Vivekananda/Volume 1/Lectures And Discourses/The Gita II

As indicated in ISH Upanishads: ~ By worshipping Gods and Goddesses you will go after death to the world of Gods and Goddesses. But will that help you? The time you spent there is wasted because if you were not there you could have spent that time moving forward towards Self-knowledge, which is your goal. In the world of Gods and Goddesses, you cannot do that, and thus you go deeper and deeper into darkness.

It clearly indicates that: ~ If the human goal is to acquire Self-Knowledge then why indulge in rituals and glorifying the conceptual Gods, Goddesses, and Gurus to go into deeper darkness. Instead,   spend that same time moving forward towards Self-knowledge or Brahma Gnana or Atma Gnana,  which is one’s prime goal. :~Santthosh Kumaar