Hinduism

Hinduism is the main religion of the Indian subcontinent. In Toaru Majutsu no Index, Hinduism and Hindu mythology are used as a base for Magic, primarily by Indian cabals.

Principles
It should be noted  that Hinduism encompasses a wide range of beliefs and is not bound by a single doctrine. By no means can the entirety of Hindu beliefs be summarized; as it is easily the most complex religion in the history of the world. For example, there is no one leader of the religion, equivalent to the Pope or the Dalai Lama, so there are different sects of the religion with different dogmas.

Contrary to popular Western belief, most modern Hindus will attest that it is monotheistic, not polytheistic. To sum it up simply, in the same way Christianity has the concept of the Father, Son, and Holy Spirit being different forms of the same God, Hinduism has a similar belief: Vishnu, Lakshmi, Lord Shiva, Bramha, Hanuman, etc. are all different forms of one God, known variously as Brahma, Ishvara, or other names. So, though Hindu Mythology contains a vast pantheon of gods and beings, they are all recognized to be part of the same Sarvasva or "All-Being", i.e, God. Of course, this is not set in stone, and debates on whereas Hinduism is polytheistic or monotheistic have occurred for centuries.

Hinduism is also very loose and allows acceptance of other religions; the practice of Hinduic ideals and beliefs, known as Sanadhana Dharma, is defined as "a way of life", and not a religion. For this reason, the very concept of conversion does not exist in any of the Hindu texts, and those that wish to convert to Hinduic faith can actually do so with no ceremony, and can even keep practising faith in another religion. However, due to modern-day inter-religious tensions, this cannot be stated as a general sentiment, and there is a significant minority that holds on to the belief that you have to be born Hindu to be Hindu.

This makes Hinduism the only major world religion that is henotheistic by nature, i.e, involving devotion to one God while accepting the existence of others. This is why most Hindu sects do not seek converts, as there is an accepted belief that the goals of spiritual life can be attained through any religion, as long as it is practiced sincerely. It is also one of the only religions in the world that allows the practice of atheism within itself.

History
Hinduism's roots can be traced back to Iron Age India, and as such it is believed to be the oldest living religion in the world.

Uses of Hinduism & Hindu Mythology in the story

 * Vishnu: The supreme deity of Vaishnavism, and part of the Trimurti together with Brahma and Shiva, representing preservation.
 * Marian Slingeneyer uses Dáinsleif to summon Vishnu Avatara, an amalgamation of Vishnu's avatars, by cutting away the Amaravati Phase.

Users of Hindu-based Magic

 * Laksha
 * Harisha
 * Gate of the Gods Come from the Heavens