We find the answer to this question by understanding how Hindus perceive God. God in Hindu scripture and in mythology takes on many forms. Yet, Hinduism understands that all of these forms are aspects of the single unfathomable power that has created, preserved, and destroyed worlds at its will. Thus, Hindus have no doubt that God is One.
It is difficult for outsiders looking into Hindu belief to wrap their heads around the pantheon of God’s appearances, God’s incarnations, and lower gods who are worshipped but are not God. In the Bhagavad Gita, the Lord says:
“Look! I am in hundreds of thousands
of different forms and colors and shapes.
Arjun, see in my body the whole world
and anything else you want to see.”
This verse invites each of us to envision God in any way we choose. The Hindu perception is that the Lord is an infinite force like a flame that emits countless sparks. He or She or It materializes as any aspect of the world, or worlds, or of the universe, or multiverses. God is beyond dimensions we know of and dimensions we have not yet understood. The Lord is not only real, but is the only reality.
The Hindu understanding of God gives rise to a distinctive worldview. For example, while Western philosophy views reality as that which can be measured and quantified, Hindu philosophy views scientific reality as illusory because it is transitory. Hinduism views true reality as that which is eternal and timeless, which transcends our understanding and which is willed in and out of existence by God who is present even in nothingness.
While Hinduism is analytically complex, it is practically simple. It is rich, colorful, and flexible. If we by-pass analysis, we can describe Hindu belief as to the overall sense that order and chaos in life exist under the auspices of a singular higher force. Through our behavior over multiple lifetimes, we can achieve enlightenment or oneness with this force called God. In the Bhagavad Gita, Lord Krishna repeatedly assures us that if we believe in Him, He will love us in return and we will merge into Him.
See On Hinduism, by Irina Gajjar, Chapter 2, Monotheism.