The diversity of religious belief and practice in India is both unique and somewhat confounding. What follows is a very brief introduction to the religions that took root in India; this will hopefully provide some insight into the patterns and diversity that exist. Tribal religions, ostensibly pagan, often mixed with elements and practices of mainstream religions, still exist in isolated pockets, but are declining rapidly and are not covered here.
Hinduism -- To begin the unending journey of studying India, you need to take the first step toward understanding Hinduism, the religion of some 80% of India's population. It can only be a "first step," for like India itself, Hinduism defies attempts to clearly define or categorize, and what may be described as universal Hindu religious practice in one place may very well be contradicted by others elsewhere.
Hinduism has no ecclesiastical order, nor is there a central religious book. (While many religious texts like the ancient Upanishads and Bhagavad-Gita exist, they are not the "word of God" as the Bible or the Koran.) It is not possible to convert to Hinduism; you are born Hindu, usually into one of the four main hierarchical castes (Brahmin, or "priest"; Kshatriya, or "warrior"; Vaisya, or "merchant"; Sudra, or "peasant") or -- at the very bottom of the social order -- you are born Dalit, better known as the "untouchables." Unlike organized religions such as Christianity or Islam where truth is specified, categorical, linear, and one-dimensional, truth in Hinduism is in fact extremely multidimensional -- contradictions are not bad, but inevitable. Unlike Christianity and Islam, which say there is one true path that leads to one God, Hinduism says there are many paths that lead to many gods (some say -- probably hyperbolically -- 330 million gods, who epitomize a host of human qualities, from gluttony to vengefulness). This intrinsic Hindu acceptance of diversity and multiplicity has defined India's history, allowing it to successfully adapt by absorbing the beliefs of successive invaders. Even today it is not difficult for Hindus to look upon Allah or Jesus as deities worthy of veneration -- more than half the devotees who flock to pray at the famous Muslim shrine in Ajmer in North India or at the fabled Velankanni Church in South India are Hindus.
Rather than a formal religion, Hinduism is considered a way of life, or Sanatan Dharma (an eternal path), in which the universe is part of an endless cycle of creation, preservation, and dissolution. The human soul is also part of this cycle, endlessly reincarnated, and seeking freedom. According to Hindu philosophy, we determine our destiny by our actions. Karma is the law of cause and effect through which individuals create their own destiny by virtuous thoughts, words, and deeds. Each of us can control the nature and experiences of the next life (karma) by "living right" or dharma -- through dharma (a righteous pattern of conduct), individuals determine their karma. By resolving all karmas, the soul can finally attain moksha, an escape from the cycle of life.
There are many sects and denominations within Hinduism, and priests, sadhus (holy men), and other spiritually enlightened individuals are important parts of the religious process: Bhakti is devotion to and communication with the gods, which devotees express in the performance of puja (religious ritual-like prayer), bhajan (devotional singing), and meditation. Puja may be performed at home or in a temple in front of an idol(s) of god(s). It involves some kind of offering to the gods (flowers being the most common) and is an essential part of the practice of Hindu faith. For Hindus the physical symbol or idol of God is the material form through which God appears in this world. Hindu devotees may worship Shiva, Kali, Ganesha, or any one of thousands of gods and manifestations of gods in the Hindu pantheon, and may believe in a supreme being who is either their chosen deity or some unnamed force even higher than the gods. Hinduism believes in the existence of three worlds: The material universe we live in, the astral plane where angels and spirits live and, finally, the spiritual world of the gods.
Buddhism -- Though Buddhism originated in India around 500 B.C., when the Indian prince Siddhartha Gautama attained enlightenment and became the Buddha (Enlightened One) at Bodhgaya, only some eight million still practice the belief in India, the majority of them from Tibet or Nepal or converted during the mass conversion of lower castes by the anticaste leader Dr. Ambedkar in 1956. (The Buddhist following is, of course, far higher outside India, particularly in the rest of Asia; even in the West, Buddhism appears to be on the rise.)
Unlike any other religion, Buddhism does not advocate belief in a godhead; it instead expects the individual to seek truth within his own experience and control his dharma and karma without relying on divine intervention. Buddhist philosophy is based on the idea that life is riddled with conflict and pain caused by desire (or craving) and ignorance, and to escape from this suffering you need to follow the Eight-Fold Path to gain enlightenment, or nirvana. The Eight-Fold Path advocates right understanding, right thought, right speech, right action, right mode of living, right endeavor, right mindfulness, and right concentration. As is the case in Hinduism, each of us carries our karma through a cycle of rebirths until the attainment of nirvana. Meditation, chanting, counting beads, and lighting lamps are some of the ways in which Buddhists pursue their spiritual goal of enlightenment.
Jainism -- This began as a reform movement and became a religion under the 24th Jain trithankara (prophet) Vardhaman, later called Mahavira (incidentally, a contemporary of Buddha), in the 6th century B.C. Though it never spread beyond India, today some four million Jains live here, predominantly in Maharashtra and Gujarat. The principles of Jainism include strict vegetarianism and extreme reverence for all forms of life -- even insects and plants are believed to have jives, or souls. Jains believe in reincarnation and salvation (or moksha), which can be achieved through respect for and consideration of all forms of life, and living a life of asceticism, meditation, fasting, and pilgrimage to holy places. According to Jain philosophy, the soul journeys through 14 stages before the final burning up of all karma and freedom from bondage.
There are two sects of Jains -- Svetambara and Digambara. Svetambara followers vow to avoid intentional injury to others and to lead a life of honesty and detachment from worldly passions. The Digambaras are even more strict in their beliefs and practices -- as a symbol of their complete detachment from material possessions, the highest monks of this sect wear no clothes. In addition, unlike Svetamabaras, Digambaras believe women cannot achieve moksha. Their temples are among the finest in India: Karnataka's famous Sravanbelagola Temple is a Digambara temple, while the celebrated Dilwara (Mount Abu, Rajasthan) and Shatrunjaya (Palitana, Gujarat) temples are important Svetambara places of pilgrimage.
Sikhism -- This religion emerged in the 15th century out of a rejection of caste distinctions and idolatry under the founder, Guru Nanak, who wanted to bring together the best of Hinduism and Islam. Nine gurus, all of whom are equally revered by Sikhs, followed him, and today there are over 19 million Sikhs in India, mostly in Punjab. Like Hinduism, Sikhism accepts the doctrine of reincarnation, but worship is based on meditation and not ritual or asceticism. Like Muslims, Sikhs believe in one omnipresent universal God; worship takes place in gurdwaras, and the holy book is the Granth Sahib. The 16th-century Golden Temple at Amritsar is the holiest Sikh place of worship (and has a truly sacred atmosphere).
Charity is an important aspect of the religion, and the gurdwaras always run community kitchens where anyone can eat free. Sikhs are expected to never cut their hair -- which makes Sikh men, who wind their long hair under large turbans and sport large beards, one of the most easily recognized male communities in India.
Other Religions -- Zoroastrianism, Judaism, Islam, and Christianity did not originate in India, but are all represented, with Islam and Christianity (which, incidentally, originated within 600 years of each other) the second- and third-largest religious groups, respectively, in India. Muslims comprise about 13% of the population, while Christians form just 2%. Every major Christian and Muslim sect and denomination is represented, and the beliefs and practices of each group vary accordingly, some with indigenous nuances. But overall they tend to follow the main tenets of these religions as practiced worldwide. It is estimated that only some 5,000 Jews still live in India, mostly in Mumbai, and these numbers continue to dwindle. Zoroastrianism, one of the world's oldest religions, dating from the 7th or 6th century B.C., arrived in India in the 10th century A.D. with refugees fleeing religious persecution in Persia (latter-day Iran). Though numerically they are a tiny religious minority (70,000), the descendants of these refugees (called Parsis) have made a distinctive mark as a social and economic group in India. Followers believe in a single God, Ahura Mazda, whose prophet Zarathustra is their guide, and fire is considered sacred and symbolic of God. Parsis therefore worship in a Fire Temple (closed to non-Parsis), and Zoroastrian philosophy regards life as an eternal battle between the forces of good and evil. Again, the path to overcome evil is through good thoughts, words, and deeds. Where possible, their dead are placed in dry wells at a Tower of Silence to be consumed by vultures. This practice is based on the belief that since dead matter pollutes, cremation and burial would pollute the respective elements. Today's Parsis regard this unique method of disposing of the dead as being useful to the cycle of life even after death.