Hinduism is a collection of various belief systems, generally related by a few unifying works of religious literature. Possibly the oldest of these are the four Vedas. More recent Hindu thought has been shaped by the Mahabharata, of which the Bhagavad Gita is a part, and the Upanishads. In 2001, there were about 820 million Hindus in India.
Sikhism started about 303 years ago -- so it started in 1699? [RFC] As of 2001 there were 35 million Sikhs.
Islam arrived through Pakistan as early as the 700's AD. During the following decades, significant numbers of Indians converted to Islam. In the 1500's, the Muslim Mughal Empire was formed. It gave India the Taj Mahal and countless other cultural enrichments to the country. Today there are about 130 million Muslims (2001), most of whom live in the north and west of the country.
Christianity, long prevalent for 1,900 years, increased significantly during the Dutch and British colonial periods. Missionaries arrived in the early 1800's along with the colonists to convert native people.
Buddhism originated in northern India and rapidly gained thousands of adherents during Gautama Siddhartha's lifetime. Up to the 9th century, followers of this way of life amounted into the hundred-millions. While the decline of Buddhism in India and its exact cause is disputed, Muslim invaders are recorded to have caused massive devastation on monasteries, statues, and libraries. Clashes with Islamic and Hindu fundamentalists are also to blame which left Indian Buddhists open to genocide & conversion. Many escaped to China, Sri Lanka (formally Ceylon), Tibet, and other Asian countries.
Recently, a revival of Buddhism in India has made adequate progress. In 1956, Ambedkar, a Dalit or untouchable (the lowest caste in Hindu society, and his followers began converting to Buddhism in protest against the caste system. Later that year he led a mass conversion of hundreds of thousands of former Dalits into the Buddhist community, or sangha. Subsequent mass conversions on a lesser scale have occurred since then. This mass conversion has added to the adherents of Buddhists in India along with the Tibetans refugees displaced by the Chinese conquest of Tibet.
Jainism also originated in India, at roughly the same time as Buddhism. A form of ancient Zoroastrianism continues as the Parsi faith.
Lately, Sathya Sai Baba of Puttaparthi is asserting the unity of all the above faiths and preaching to embrace love.