He looked at me with slightly glazed eyes through the pulse of bodies on the banks of the Ganges.

"Hinduism is as big as your mind or as small as your mind," said the smiling sadhu, stroking a belly that was as swollen and smooth as a spacehopper. Well, that seemed to wrap it all up really, one of those great throwaway lines that the wandering holy men of India, the sadhus, know the foreigners want to hear and will cogitate over for hours in the shimmering heat of the subcontinent. But beneath the glaze- eyed guru gimmick was the nut of Hinduism.

It is a huge religion, the oldest in the world, with a confusion of thousands in the Hindu pantheon of gods. And even if you were to crack the caste of thousands, each one of them has a vehicle, an animal of some description, that flies or trundles them around the heavens on their otherworldly missions. It is this hugeness that is daunting, but Hinduism is an onion religion that is heavy with the ritual paraphernalia attached to it by the Indian nature and culture. As you peel away the layers you'll find the simple moral codes of human behaviour that are at the root of Hinduism. If you dig down through the extraordinary scriptures to one of the main works, the Upanishads (400-200 BC), you'll find a text that incorporates the central theme of the majority of the world's main religions: 'The Great God is One, and the learned call him by different names.'

Hinduism's thousands of gods lead to the big three - Brahma the creator, Vishnu the preserver and Shiva the destroyer (sometimes also seen as a creator) - three in one, and also to the physical representations of the unseen omnipotent God, Parabrahma.

When the glaze-eyed sadhu with the spacehopper belly said that it was "as big as your mind", he was referring to the ability of Hinduism to embrace so many forms of worship; every family has a favourite god or goddess to whom they turn for guidance, support and comfort. But from out of all the layers of belief comes one core creed: the acceptance of Samsara, Moksha, Karma and Dharma; Samsara, the cycle of rebirth through many lives on the way to attaining perfection; Moksha, spiritual salvation and release from the cycle of reincarnation; Karma, the law of cause and effect, whatever you do, good or bad, there will be a consequence and crimes and good deeds that are not recognised in the current life may be punished or rewarded in the next; and Dharma, the natural balance of the universe, the law of the caste system and the moral code that each person should follow.

For the visitor to India and Asia, whether travelling through Hindu societies or living in them, a basic understanding of the caste system is important. The control of the caste system has been profoundly challenged for centuries, most dramatically in 1947 when Jawaharlal Nehru became the first prime minister of independent India and called for secular government. Nehru's hope, to form a government free of religious undertone, was an idealistic cry for a newly independent nation that has always ebbed and flowed on the tides of religious passion. The caste system is deeply ingrained in the Indian psyche and, even though it no longer officially exists as a class structure, the four main castes still affect daily life at every level. It has become largely diluted in the cities but it remains the rule of thumb in most rural areas. To ignore it is to bypass a great chunk of India at its pulse.

There are two things about Hinduism that were historically set in stone: you were either born a Hindu with a caste or you were not, and you could not truly convert to Hinduism. This may well be one of the reasons that foreigners have found Hinduism so fascinating, because ultimately we could find out as much about it as we liked, we could read the Mahabharata, the Bhagavad Gita, the Ramayana, the Vedas, the Upanishads and the Puranas, we could go to puja prayer rituals, we could pray with Hindus, sing their mantras, accept the blessing of their priests, but we could not claim a caste and convert, in spite of the openness that the religion offered to non-Hindus. Aspects of that are changing as times change and, increasingly, people marry across religions. Within its own fluidity, Hinduism has become more relaxed about conversions beyond the caste system.

So from the big stuff of gods and systems to the smaller but equally important stuff. Hinduism is a living religion in that it is a part of the daily round and there are some things that need to be understood.

Some people think that the Indians are prudish, that they cover up and hide everything. By Western standards they do, but there is a very beautiful sensuality in that very covering. When I work in India with men fresh out of England, when they catch their first sight of sari-clad bathers in a river or beside a village tank, the material wrapped and clinging to their curves, their response has always been to be awed by the feminine beauty. Hindus find it difficult to deal with the Western idea of stripping down to as little as possible in a hot country. If you have ever lived through a hot season in India you become only too aware of the fact that loose, flowing, light cotton clothes are the coolest and most practical. You just get burnt and bitten in shorts and T-shirts.

I had one of my closest shaves with the early edges of middle age when I was taking a young American student out to a primary school project located in a Delhi slum. She was gorgeous, blonde and curvy, and she turned up wearing a sleeveless shirt that showed a fantastic display of cleavage and a flippy skirt. She challenged my disapproval with a tone of voice that made me feel 30 going on 90. She had picked up the skirt in a local market without realising that it was actually a sari petticoat. She was unintentionally breaking every code of decent dressing in Hindu eyes. She was wearing underwear, for a start, and she was also displaying her upper arms and cleavage. All that is fine and acceptable in the nightclubs and bars of Delhi and Mumbai, but it is just not fair outside of these cities, particularly in rural areas. It is confusing for the people and unfortunately helps to reiterate the belief held by many rural and village Indians that all white women are basically asking for it.

When it comes to going into a temple or attending a family prayer puja, both men and women should cover their heads, arms and legs. No naked flesh is the easiest rule to follow. You will almost always be asked to take your shoes off before going into a temple by an official shoe-minder. It is worth checking that you are handing your precious shoes over to the official minder rather than a likely lad who is going to skip off down the road with his booty, leaving you hopping around on boiling roads where rats and broken glass loiter. The official minder should be tipped a few rupees for his labours.

Weddings have a dress code all of their own. My pet theory is that it is wise for foreigners is to stick to Western dress, but keep it simple and elegant, and again no exposed flesh. Hindu weddings are all about glitter and show and there is just no point in trying to match the efforts of the native population. I don't know why but it always make me a bit sad seeing Western girls struggling with saris at weddings or parties. Indian women were born to it and carry this exquisite garment with grace. Most Westerners just get it a bit wrong and would look much more elegant and have more fun if they stuck to their own clothes. I've lost count of the number of Western girls and women that I have seen in glitzy hotel loos tacking saris up with safety pins, and looking totally miserable about their failure to float within the folds of silk, while their sinuous Indian counterparts waft on by.

While on the female take, historically and culturally Hindus have great respect for women - one of their favourite deities is the gorgeous Lakshmi, goddess of wealth and prosperity and consort of Vishnu, the preserver in the big three. But if a woman does not carry herself within the cultural boundaries of Hindu sensibilities men will understandably regard it as OK to leer and provoke to their hearts' content. If you give them something to stare at, they will stare and go on staring.

Hindus do not eat beef. The cow is sacred to them, hence the vast numbers of cows, decorated with gaily painted horns, wandering around the streets, half-chewed plastic bag in mouth, looking not unlike fag-ash Lil on jaunty hat day. Many people are vegetarians, but just as many eat meat - mutton and chicken being the most popular.

The eating implement of India, and most of Asia, is the right hand. If you are eating with locals and joining in with your hands, too, don't use the left, it is reserved for the washing of the bum. To use it for eating is both offensive and embarrassing for your hosts or fellow eaters.

One further warning about partying with Hindus, or more generally Indians: theoretically most of them do not drink as it is against some aspect of their religion, but at flash weddings or parties the booze flows freely and for a long time. And the drinking goes on until about midnight. Then, when most people are very blurred around the edges, a great dinner is produced. Everyone eats hugely and then rolls off home repeating the Indian late-night chorus of indigestion. If you get hungry early, eat before you go and nibble politely at the midnight binge.

Hinduism is a bright, brilliant culture and religion that, like its partying, can seem indigestible if taken on too quickly but, when absorbed slowly, is as deep and voluptuous as the Ganges - or your mind, as the man with the spacehopper belly said.