In reality "God is One" who but appears in different forms. All these Gods & Goddesses are manifestation of one Supreme God only. It can be understood very easily like this:
"There is a potter who makes different forms of Gods & Goddesses from clay of the ground. Clay is basically the same in all the forms but out of same clay different Gods & Goddesses appear to us. In the same way there is only One God but it appears in different forms. Similarly if you see that Gold for making different ornaments is same but with this gold various types, shapes of ornaments are made & are given different names. Similarly God is One but He is worshipped in different cultures, countries in different ways, forms & with different names. There are different paths in this world to reach that same One God."
In Bhagwat Gita Chapter Ten, Lord Krishna is himself saying that He is the source of all beings & gods. Not even that, as per Lord Krishna, all these universes & creations spring from but a spark of His splendor only. As per Him, He is seated in the hearts of all living entities. He Himself is Lord Vishnu, Lord Shiva & Lord Brahma. He Himself is Sun, Moon, Wind & Fire. He Himself is Rama God too. He Himself is Lord of wealth - Kubera. He Himself is Spiritual word Om. He Himself is river Ganges. Among women He Himself is manifested as Goddess of fortune (Lakshmi), Goddess of wisdom (Saraswati) & other Goddesses of fame, memory, intelligence, steadfastness and patience. As per Lord Krishna His opulence is limitless.
Hindus likewise believe in only one God. They glorify and praise His thousands of qualities. For example when God is love, He is symbolically represented by Lord Krishna, God as virtue is represented by lord Rama, God as power is represented by Lord Hanuman, God as wisdom is represented by Goddess Saraswati etc. etc. So these different aspects of Gods represent spirituality, purity, power etc. of the Ultimate One who is endless truth and bliss.
If there are many "Pots filled with water" & if these pots are put under the Sun then Reflection of the Sun can be seen in all these pots. It appears that individual Sun is appearing in each pot. Not only that, water of each pot gets heated equally when Sun is reflected in the water pots. Now just as the Sun may be reflected in numerous pots of water, so the same one God is reflected in everything. Similarly there is one True Universal Self or One God who is appearing in different forms.
Below are some verses from Bhagwat Gita which explain about the Oneness of Supreme God:
"yac chapi sarva-bhutanam
bijam tad aham arjuna
na tad asti vina yat syan
maya bhutam characharam" (Bhagwat Gita: Chapter Ten verse 39)
"Sri Krishna said: O Arjuna, I am the generating seed of all existences. There is no being—moving or nonmoving—that can exist without Me."
"aham atma gudakesha
aham adis cha madhyam cha
bhutanam anta eva cha" (Bhagwat Gita: Chapter Ten verse 20)
"Sri Krishna said: I am the Super Soul, O Arjuna, seated in the hearts of all living entities. I am the beginning, the middle and the end of all beings."
"yad yad vibhutimat sattvam
srimad urjitam eva va
tat tad evavagaccha tvam
mama tejo-’msa-sambhavam" (Bhagwat Gita: Chapter Ten verse 41)
"Sri Krishna said: Know that all opulent, beautiful and glorious creations spring from but a spark of My splendor."
So let's see one God in all different forms. Let people not fight on the basis of religion. There is only one God & He is called by different names by his lovers, devotees in affection. Sri Ram, Sri Krishn, Gobind, Waheguru, Allah, Jesus are all the same. So let's all have wisdom to respect each others faith & let's all realize that One God in all the different forms.