From its beginning, Hinduism has been undergoing evolution. At a very early stage of their civilization, the ancestors of the Hindus are believed to have been polytheistic. Earth, water, fire, wind, sky, sun, dawn, night, thunderstorm – all were deified and adored as gods. But while being praised by the Vedic hymns, each of these gods was addressed or referred to as the Supreme God, the Lord of all gods, and the Creator of this universe. According to the famous German Indologist Max Muller, the earliest ancestors of the Hindus were, therefore, not polytheistic; they were henotheistic.
Gradually the Indo-Aryan mind discovered some common ground behind this multiplicity of gods. The Nasadiya Sukta or the “Creation Hymn” of the Rig Veda tells us in beautiful and poetic language about a single primordial and extremely abstract principle designated THAT, from which the entire world has evolved. This principle is Pure Consciousness or Pure Spirit. It is beyond the world of space and time, beyond multiplicity, unfathomable and unknowable by ordinary human minds. That principle was there when neither the gods, nor men, nor anything else in creation existed. From that One and Only principle, the world of Many has evolved. The Indo-Aryan genius, at last, arrived at the One and Only cause of everything, the One and Only God, who in Vedic Sanskrit is called Brahman. After that divine revelation, the Vedic texts echoed the truth of the Oneness of Brahman again and again.
Vedic statements like “Ekam sad vipra bahudha vadanti” – “One alone exists, sages call It by various names,” not only emphasize the Oneness of God but also form a firm foundation of catholicity and tolerance in Hinduism. The idea of harmony or religions is a fundamental ingredient of Hinduism.
The great sage Manu declared, “One ought to know the Supreme Spirit Who is the Ruler of all, subtler than the subtlest, of resplendent glory, and capable of being realized only by the meditation of pure-minded ones. Some call Him Agni (Fire); others call Him Manu (Thinker); and others Prajapati (Lord of creatures). Some again call Him Indra (the Glorious); others Prana (the Source of Life); and still others the Eternal Brahman (the Great).”
Source: Swami Bhaskarananda, The Essentials of Hinduism,Sri Ramakrishna Math, Chennai, page 65-66