There are thousands of religions across the world. Most of these religions believe in the concept of God. Few religions like Jainism, Budhism does not believe in the concept of God. Some religions like Christianity, Islam believe that there is only one God. And some other religions like ancient Egyptian religions, ancient Greek religions believed in multiple Gods and Goddesses. In Hinduism it appears deceptive, because they worship many Gods and Goddesses, and at the same time say that there is only one God! How this can be possible and make sense; we will try to find out.
Most of the religions have one sacred or holy book like for Christians Bible is the holy book and for Muslims Quran is the holy book. For Hindus Bhagavad Geeta is one of the holy books. Even though Bhagavad Geeta is more commonly referred book by Hindus, there are many more holy books such as Vedas, Upanishads, Bhagavatam, etc. Unless one studies all of them carefully it is not possible to comprehend Hindu philosophy in totality and in sensible perspective.
Hindu scriptures explain that God is beyond human perception and understanding. Any of the human sense organs are not capable of perceiving God. Like, even though we know there is X-ray, neither our eyes nor any of our other sense organs are capable of seeing it or perceiving it, but we know about it through scientific instruments. No human language is evolved enough, and there is no vocabulary available to explain God. So one should first understand that whatever way and however we try to explain God, that is only a nearest possible depiction within the capabilities of our language and perception, which may not be perfect, but may be some what like that.
In some religions it is believed that God is the creator of the universe. He created land, rivers, sea, trees, animals, people and all surroundings; and all these belongs to Him.
As per Hindu scriptures the Supreme Lord, who is absolute and without beginning expands himself
into unlimited forms, and manifest in different forms. The whole universe and the world we see including land, sea, trees, animals, people and all the surroundings are manifestation of the Supreme God. So everything that exists is pervaded by God or manifested form of the same Supreme God. That's why they say there is only one God but his expressions are many. That formless Supreme God manifests in many different forms like divine beings and all that exists in the universe like Sun, Planets, Wind, Trees, Animals etc.. With this belief Hindus worship not only many divine beings like Lord Rama and Lord Krshna whom they call Gods but also worship nature like Sun, Trees, Snakes, etc.. Here the Supreme Lord is impersonal, formless pure energy or divinity. The manifested forms like Rama, Krshna are incarnations with form. If you compare a formless lump of clay to the Supreme God, then the bricks, tiles, pottery, etc. are different expressions, made out of the same lump of clay, which are like manifested forms of God. Some times this Supreme Lord is also referred as 'Brahman' [ब्रह्मन्]. There are few different meanings for the word Brahman according to context, so one should be careful to interpret according to the context. Then again word 'Brahma' [ब्रह्म] is different from word 'Brahman'. When we talk about God apart from Supreme God, two more phrases used are Bhagavan [भगवान्] and Paramatma [परमात्म]. Here Bhagavan refers to incarnations of Supreme God like Bhagavan Rama, Bhagavan Krshna and other divine beings. Pramatma is the Supreme God in the form of spirit that dwells in all living beings; when this spirit departs from the body we say that living being is dead. So the Supreme God is formless, pure energy, and his manifestations like Lord Ram or Krshna are in human form.
While Hindus worship many Gods/Goddesses, it is not necessary for them to believe in many Gods or even one God. But it is very important to respect nature and follow the law of nature and not to disrespect nature and tamper with it; Because they believe nature is manifestation of God and divine pervades everthing in nature. And it is also very important for Hindus to be morally right and do good. If you do good you will have good results and bliss, if you do bad you will get bad results and suffering. One of the important belief is 'law of karma' that is law of cause and effect which is law of nature. So if you also believe in that let us keep doing good.
Now let us look at few verses from the scriptures related to these concepts.
41st Verse of Chapter 10 in Bhagavd Geeta 
yad yad vibhutimat sattvam shrimad urjitam eva va
tat tad evavagaccha tvam mama tejom sasambavam.
Which means 'All glorious, prosperous, powerful and mighty creations/beings are a manifestation of a fragment of my splendor.'
2nd Verse in Chapter 10 of Bhagavd Geeta
na me viduh suraganah prabhavam na maharshayah
aham adir hi devanam maharshinam ca sarvashah.
aham adir hi devanam maharshinam ca sarvashah.
Which means 'Neither demigods nor the great sages know my origin and opulence
[secret of my manifestation in human and all other forms], for, in
every respect, I am the source of the demigods and the sages.'
39th Verse in Chapter 10 of Bhagavd Geeta
na tad asti vina yat syan maya bhutam caracaram.
Which means 'I
am that which is the seed of all beings. There is no creature moving or
unmoving, animate or inanimate that can exist without me.'
42nd Verse in Chapter 10 of Bhagavd Geeta
vishtabhyaham idam krtsnam ekamshena sthito jagath.
Which means 'I pervade this entire universe. I support and sustain this entire universe with a fragment of mine [myself].'
4th Verse in Chapter 9 of Bhagavd Geeta
mastani sarvabhutani na caham tesvavasthitah.
Which means 'This entire universe is pervaded by me, in an unmanifest form. All beings, animate and inanimate, exist in me, but I am not present in them.'
The statement 'All
beings, animate and inanimate, exist in me, but I am not present in
them' may be confusing to some. This is like saying all waves exist inside
sea, but sea is not present in side waves. Probably this comparison might help to interpret it better.
ये यथा मां प्रपद्यन्ते तांस्तथैव भजाम्यहम्।
मम वर्त्मानुवर्तन्ते मनुष्याः पार्थ सर्वशः ॥४-११॥
मम वर्त्मानुवर्तन्ते मनुष्याः पार्थ सर्वशः ॥४-११॥
ye yatha mam prapadyante tams tathaiva bhajamy aham
mama vartmanuvartante manusyah partha sarvasah.
mama vartmanuvartante manusyah partha sarvasah.
Which means 'By whatsoever way people seek me, I accept them the same way. Every one follows my path, knowingly or unknowingly.'
Thus there is one God who appears to us in the form that we seek him in. He appears as both niraakar [निराकार्] that is formless, and sakaar [सकार्] that is with form. The logic being that the God who manifest as the universe, can also assume any form in anyway, anywhere, and simultaneously at many places.
7th verse in Chapter 4 of Bhagavad Geeta
Yada yada hi dharmasya glanirbhavati bharata
Abhyuthanamadharmasya tadatmanam srjamyaham
Abhyuthanamadharmasya tadatmanam srjamyaham
Which means
'Whenever and wherever there is decline in moral values or
righteousness, and predominance of immorality or unrighteousness, then
at that time I will manifest.'
Please note, the last word in the verse [सृजाम्यहम्] srjamyaham is combination of two words [सृजामि] srjami + [अहम्] aham. Here srjami means 'manifest' and aham means 'I'. In English it means 'I manifest'. It is important to understand that the word srjami
is not used to say create, it is used to say manifest. Hindu scriptures
does not say God created the universe. It says God manifest himself as
He is in different forms, and the whole universe is pervaded by God and every thing that
exist is nothing other than the part of God.
In a tree there are many branches, many leaves and many flowers. But all these grow from one and the same trunk of a tree, even though they look different. Similarly if you get into proper interpretation of teachings of different religions, almost all religions trace to one God, the source of Universe. So the teachings and prescriptions of all religions should be to unite all of us, not to divide and spread hatred among us. Need of the hour is to encourage religious preachers who could interpret scriptures correctly in right perspective and spread the message of love and harmony.