Worshipper,
Seyorni, has not been posting here for a while, so I hope you don't mind if I answer your questions.
I want to first say I am an objectivist. This is meaning to say I make statements of fact. If I say that in Hinduism there is only one supreme being, I mean that as a fact, even if others do not accept that or interpret differently. This is because I define Hinduism as being a Vedic religion and only that which is in consonance with the Vedas is Hinduism. This has also been the traditional definition of Hinduism, Hinduism has held the Vedas to be the supreme authority of Hinduism, anything which goes against the Vedas or diverges from the Vedas is rejected from Hinduism.
I want to give you a rough idea of the evolution of Hinduism. Hinduism was founded by hundreds of different Seers(Risis) some 10,000 years ago(the latest research indicates that they are the oldest books in history) and they existed in the form of an oral tradition at first, but were penned later. These Risis were said to have experienced ultimate reality and fathomed the deepest secrets of the universe in their meditations. They recorded the knowledge they received in the form of hymns, glorifying many deities. They laid down a path called Santana Dharma(the eternal religion of universal laws) and used this to develop a spiritual society which was in harmony with universal laws. This was a very prosperous and advanced society as can be seen from archeological remains.
This Vedic religion flourished for thousands of years in India, but then due to geological changes, there was a decline and the Vedic age ended around 1000BCE. This was the start of what is called the Classical age. This is the era in which Buddhism arose and the Hindu Philosophical schools. The Hindu Philosophical schools all accepted the authority of the Vedas and were known as the Astik schools; Buddhism rejected the authority of the Vedas and was known as the Nastik school. The Hindu schools expanded on Vedic teachings to create rational schools of philosophy. Then later on the epics Mahabharata, Ramayana and the Gita were composed. The Gita is considered the quintessence of the Vedas and is the most sacred texts of Hinduism.
The period up until now is considered the golden age of Hinduism. It is at its most purest and virtually all of Hinduism's central texts come from this period. Later in the early middle ages corruption starts to sets mainly due to invasions etc and Puranas are composed(they are allegedly older, but the first records of the Puranas are in the 3rd-5th century CE) This could be called the Puranic period. These are written in the form of myths, stories, geneologies and histories of the older Vedic times to teach to the masses, as the Vedas were to difficult to understand by the masses. They remained in the public domain, and were regularly altered over the years. This is when the polytheistic culture emerged. In this period caste-system become more rigid and oppressive, womens lost a lot of their freedoms, society became patriarchal and theocratic. However, the Vedic tradition was still kept alive amongst the intelligensia in the various universities that existed in India(Nalanda etc) but there was a decline in society.
In the 8th century AD there was a huge revolution in Hinduism; the birth of Adi-Sankacharya who resurrected the Vedanta tradition of yore and founded Advatia(non-dualism) and a sect of Hinduism known as Smartism. The non-dual school of philosophy proposed by Sankacharya was revolutionary then. Sankacharya propounded, "Shivo Aham" , "Aham Brahasmi" - "Shiva I am", "I am Brahman" that there is only one real reality, the supreme being, the world is nothing but an illusion and the soul is actually the same as the supreme being but is in ignorance. The only way to attain this is through the path of knowledge(Jnana Yoga)
While Adi Sankaras teachings had caused a revolution in the religious thinking of the time, but it had also lead to people believing that because they are god, they don't have to do anything. This lead to the next huge revolution in Hinduism: Dvatia(dualism) founded by Ramunajacharya in the 11th century AD. Ramuncharya believed that the world was real, souls were real, and the supreme being was real. The supreme being is independent but the supreme controller of the entire universe. He also believed there were heavens, where angels(Devtas) resided and the supreme being himself resided in the highest heaven. He advanced the path of devotion and servitude to god(Bhakti Yoga)
This path of Bhakti became the most influencial and common path of Hinduism from thereon. It gave rise to many great saints, who loved god with unending devotion and they become the most adored amongst the people. This is the period when temple worship and the worshipping of gods exploded. The scriptures that appeared during this period are known as the Agamas, and are considered as revealed as the Vedas by their adherants.
The principal sects of Hinduism existing then, and still the main ones today are: Vaishnavism, Shivaism, Shaktism and Smartism. In Vaishnavism, Vishnu or Krishna was accepted as the supreme being; in Shivaism, Shiva was accepted as the supreme being; in Shaktism the Divine Mother was accepted as the supreme being. In Smartism, one had a choice. In all cases the understanding was there was only one supreme being, only each sect had a favourite. Although all the sects had very diverse beliefs and practices, they shared a lot in common and all accepted the authority of the Vedas and Vedic religion.
Later on more and more sects formed in Hinduism, today there are hundreds if not thousands of sects, each with very diverse beliefs, but very much a part of the same Vedic religion.
Then with the invasion of Muslims and then the British, Hinduism suffered immensely and Hindus could not practice their religions, especially under the British, as they had been reduced to poverty. In the 19th century the British brought thse hundreds of sects under the umbrella term of Hinduism. Although Hinduism existed prior to that under the name Santana Dharma or the Vedic religion. Several generations had passed in poverty and illiteracy, now ignorant of Hinduism. Then towards the end of the 19th century the British introduced subjects like Indology and Sanskrit studies and had the Vedas translated into English and mistranslated and misinterpreted making them seem like the rambings of a barbaric people and Hindus lost faith in them. Today, almost 80% of Hindus have not read the Vedas. A smaller percentage, but still significantly large, has not read the Gita. Instead they learn about Hinduism from the stories of Puranas from childhood and indulge in superstition and worship of many gods.
If you ask anybody learned in Hinduism in any sect they will tell you that there is only supreme being, only they know it by various names. If you ask somebody who is ignorant of Hinduism, they will tell you there are many many gods in Hinduism and will tell you the myths of the Puranas as if they really happened. Generally, the more one is versed in the Vedas, the more authentic their understanding on Hinduism.
