Sindu River, became Hindu river and people who lived on the other side were called Hindus by people who were west of India.
The same Sindu river became called as Indus which gave the country the name Indian.
So when you say Hindu you actually are using an etymological equivalent of India
Hindu = Indian, literally.
When referred to the religion and philosophy, Hinduism technically speaking is a group of relgio-philosophies that originated and developed in Indian subcontinent.
You can be a Hindu atheist called Charvakas amd Lokayatas who are materialists who believe in Science and Logic. And don't have any supernatural beliefs. These are Hindu Atheists. These are materialists.
The rest are spiritualists.
Three simple things are common in all of them.
- Beleif in Reincarnation
- Beleif in Karma, or consequences for your action, either in this birth or in the next.
- Belief in Dharma, or cosmic law which guards Law of Karma.
If you believe these you can be called as a Spiritual Hindu. This seems to be the bare minimum.
Then you can be a Heterodox Hindu, who are not often called Hindus, but are still Hindus technically speaking.
- Buddhists - No belief in God or in Soul
- Jains - No belief in God but believe in Soul
Then there is classical Orthodox Hinduism, who are generally called Hindus.There are six philosophies based classifications of them. They go in pairs. These are largely. Theistic, can be Agnostic.
Nyaya and Vaisheshika - Accepts Vedas but stresses on logic.
Sankhya and Yoga - Partial acceptance of Vedas. Goes by Sutras and Karikas of different Saints like Kapila Maharishi and Patanjali Maharishi etc.
Purva Mimamsa and Uttara Mimamsa - Ultimate authortiy of Vedas. Accepts Smritis which do not contradict Vedas.
Purva Mimamsa is more ritualistic and believed that Vedas are metaphorical and unauthored. This was practiced by Kumarila Bhatta - Followers called Bhattas and Prabhakara - Followers called Prabhakaras. These have largely gone extinct.
As of today, all others have gone extinct apart from Uttara Mimamsa which has incorporated some parts of Yoga and Purva Mimamsa.
Then this Uttara Mimamsa itself practiced differently by different people lead by different Acharyas. Most Hindu today knowingly or unknowingly belong to these traditions. Uttara Mimamsa stresses on Vedanta or Upanishads as containing summary and true intent of Vedas. Along with Bhagavat Gita and Brahma Sutra of Shri Veda Vyasa they form what is known as Prastanatrayi. The Smritis and Puranas and Ramayana and Mahabharata all are considered and believed.
There are different forms
- Shri Adi Shankaaracharya - Advaita
- Shri Ramanujacharya - Vishistadvaita
- Shri Madhwacharya - Dvaita
- Shri Chaitanya Mahaprabhu - Achintya Bhedabheda etc
These are all classical Hindu forms philosophocally, there are other classical Hindu forms which won't fit neatly into these category of six philosophies. They can be categorized by the God they worship
Shaivism believes in Shaiva agamas. Different forms exist like Veerashaivaism-Lingayatism, Kashmirishaivism, Pashupata, Maheshwara, Tamil Siddha - Shaivism, The Nayanars etc
Vaishnava believes in Pancharatra, Bhagavata and Vaikhanasa Vaishnava agamas. Different forms exist like Vaishnavism and Shri Vaishnavism, Gaudiya Vaishnavism, Bhakti tradition, The Alwars etc
Shaktism beleives in Shakta Agamas. There are many schools and tantric branches.
There are others which won't fit easily and have traits of many philosophies and sects like Nath Tradition and Siddha tradition.
There are others which are Theistic, but seem to be non-Vedic, like Lingayatism - Shakti Vishistadvaita, etc.
Also there are many sub religions which are formed around local avatars of God. You can find such local sub religions, where they have a main religion and one local avatar of God or a Yogi or a Guru and have a sub religion.
All these sects believe in selected relavant Puranas and Upanishads. Also philosophy based classifications and God based classifications have common sects in them. Like permutation and combination of them.
And then there is Sikhism, which many people think is different than Hinduism. But has many similarities and contains same ideas. They believe in Guru Granth Sahib and their Guru's words.
These are all classical form of Hindusim. But as you can see it is very diverse. The list I have given is very brief. Different sects believe in different things.
An average Hindu normally doesn't know what he or she practices. He or She kind of practices a superficial version of many things mixed at once. This led to neo Hindu-Yogic tradition and many modern amd contemporary schools came up. Often some divine person is born and he starts off a particular sub religion and acts as Guru. There are so many
Some of the prominent ones are
- Sai baba
- Ramakrishna Mission - incorporates Jesus and Abrahamic faith
- Ramana Maharishi
- Paramhamsa Yogananda - Kriya Yoga tradition - incorporates Jesus Christ and Abrahamic faith, has a translation and interpretation of New Testament.
- Maharishi Mahesh Yogi Tradition
- Kashmir Shaivism - Lakshman Joo
- Inchegeri Sampradaya - Nisarga Dutta Maharaj
- Sadhguru etc
There are so many others.
You will see that these philosophies, sects, practices etc are all intertwined, constantly interacting, absorbing each other. That it becomes so hard to categorize and to define who exactly is a Hindu and what he or she believes in. So a Hindu normally believes in the first three things I mentioned and then can pick anyone of the above which he or she finds comfortable with and start following and practicising.
You can go for classical, semi classical, modern, freestyle whatever you are comfortable.
But beware of doing window shopping and jumping here and there without seriously working on any path. Shri Ramakrishna compared such people to a man who digs multiple shallow wells and doesn't find any water, instead of digging a well in one place and finding water. He just wastes time and effort.
This is a very flexible and accommodating religion. Believed to follow eternal dharma, hence the name Sanatana Dharma.