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In Hindu Dharma, God is One.
“Ishvara” means That which is omnipotent (sarva-shaktimaan). Two omnipotent beings cannot exist, because it is not logically possible: if there were two omnipotent powers, each would limit the other. Therefore, like the other major religions, Hindu Dharma also accepts that God is One. He is called Brahman — or Para-Brahman.
That one Para-Brahman Parameshwara reveals Himself in many forms at the time of creation.
This raises a sensitive and beautiful question — if God is one, why does He have so many forms? Is He really formless, or with form?