Concept of God in Vedic religion

येन द्यौरुग्रा पृथिवी च दृढा येन स्व स्तभितं येन नाकः । यो अन्तरिक्षे रजसो विमानः कस्मै देवाय हविषा विधेम ॥यजुर्वेद ३२:६॥

जिस परमात्मा ने द्युलोक को तेजस्वी बनाया, जिसने सुख और आनन्द की प्राप्ति के लिए पृथ्वी को दृढ़ बनाया और आदित्य मण्डल एवं स्वर्गलोक को स्थिर किया; जिसने आकाश में नाना लोकों का निर्माण किया, उस आनन्दस्वरूप परमात्मा की भक्तिपूर्वक अर्चना करते हैं (उसके अतिरिक्त और किसकी अर्चना की जाए?)॥६॥

6. By whom the heavens are strong and earth stands firmly, by whom light's realm and sky-vault are supported; By whom the regions in mid-air were measured. What God shall we adore with our oblation?

 The concept of God in Hinduism is a little bit confusing. Many islamist scholars like Zakir Naik quote some verses from Hindu scriptures and try to prove that idol worship is prohibited in Hinduism and there is only on god in Hinduism. They do so to make innocent hindus fool and convert them in Islam. Let's see what verses they quote;

Atharvaveda 13:4:12. This conquering might hath entered him, He is the sole the simple One, the One alone.

Brihadaranyaka Upanishad VI-ii-1: 'In the beginning, dear boy, this was Being alone, one only, without a second. Some say that, in the beginning, this was Non-being alone, one only, without a second. From that Non-being arose Being.'

Rigveda 6:45:16. Praise him who, Matchless and Alone, was born the Lord of living men, Most active, with heroic soul.

Yajurveda 32:3. There is no counterpart of him whose glory verily is great. In the beginning rose Hiranyagarbha, etc. Let not him harm me, etc. Than whom there is no other born, etc.

Muslims fanatics like Zakir Naik show these verses and try tos how that all those gods whom hindus worship are fake. But these people don't have any knowledge of scriptures. Yes hindus believe in one and only one god who is called Parabrahma. He is only one and bodiless.

Mundaka Upanishad I-i-6: (By the higher knowledge) the wise realize everywhere that which cannot be perceived and grasped, which is without source, features, eyes, and ears, which has neither hands nor feet, which is eternal, multiformed, all-pervasive, extremely subtle, and un-diminishing and which is the source of all.

Yajurveda 40:8. He hath attained unto the Bright, Bodiless, Woundless, Sinewless, the Pure which evil hath not pierced. Far-sighted, wise, encompassing, he self-existent hath prescribed aims, as propriety demands, unto the everlasting Years.

Shwetashwara Upanishad 4:20. His form does not stand within the range of the senses. No one perceives Him with the eye. Those who know Him through the faculty of intuition as thus seated in their heart, become immortal.

Katha Upanishad 1-III-15. By knowing that which is soundless, touchless, formless, un-decaying, so also tasteless, eternal, odourless, beginningless, endless, subtler than Mahat and constant, man is liberated from the jaws of death.

Now the question arise is that if god is formless and only one, who are Brahma, Vishnu etc. mentioned in vedas? Is worshipping their idols wrong? Hinduism believes that God or Para Brahma incarnates in different forms to perform different functions. When Hindu scripture say that God is formless and only one, they are actually referring to Parabrahma. And when they praise Shiva, Vishnu, Brahma etc. they are actually praising the same Para Brahma in his different forms. 

Svetashvara Upanishad 4:1 May that Divine Being, who, though Himself formless, takes various forms in different ways with the help of His own power, for His own inscrutable purpose, and who dissolves the whole world in Himself in the end - may He endow us with good thoughts!

Brihadaranyaka Upanishad also talks about two forms of god.

Brihadaranyaka 2:3:1 Brahman has but two modes - formless(amurta) as well as in from from(murta), mortal and immortal, limited and unlimited, defined and undefined.

 So it is clear that Para Brahma takes incarnation as Brahma, Vishnu and Shiva to perform various functions as written in;

Bhagwad Gita 13:17.He is indivisible, yet He appears to be divided amongst living beings. Know the Supreme Entity(Para Brahma) to be the Sustainer (Vishnu), Annihilator(Shiva), and Creator of all beings (Shiva)

Mahabharata Vana Parva Section CCLXX. The Supreme Spirit hath three condition. In the form of Brahma, he is the Creator, and in the form of Vishnu he is the Preserver, and in his form as Rudra, he is the Destroyer of the Universe. 

Padma Purana 1:2:117. The creator creates himself, and Viṣṇu protects (the world) that is to be protected, and the lord himself, the with-drawer, is withdrawn (into himself).

Vishnu Purana 2:1. Thus the one only god,  takes the designation of Brahmá, Vishńu, and Śiva, accordingly as he creates, preserves, or destroys.....

Garud Purana Chapter 23. “He who is Shiva is Hari and Brahma.”

Nirukta 7:4 This is to be clearly seen (by the following) : On account of the supereminence of the deity, a single soul is praised in various ways. Other gods are the individual limbs of a single soul. Or else, as people say, seers praise objects according to the multiplicities of their original nature, as well as from its universality. They are produced from each other. They are the original forms of each other.They are produced from (action (Ax/rma)), they are produced from the soul. Soul is even their chariot, their horse, their weapon, their arrows ; soul is indeed the all-in-all of gods. 

 

Conclusion:- Hindus worship one and only one god who takes several forms.