Sanatana Dharma is a comprehensive system of life, consisting of religion, spiritual philosophy and knowledge system, and social and political setup.
Goals
Simply put, the goal of Sanatana Dharma is Moksha, liberation. It aims at Mukti for every individual, and all the methods it prescribes are towards that goal. Fulfillment of the purposes of life is the means to it. However, the phenomenal world is diverse, and hence there are diverse means to fulfilling the purpose of life.
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Premises
According to Sanatana Dharma, each being is potentially divine. Realization is about unveiling divinity. God, who created the universe, is not different from it - He exists unmanifest, but the entire creation is also a part of Him. He exists essentially, in each particle and phenomenon of creation.
Each being, through its cycles of birth and death, is evolving towards the same goal, according to Sanatana Dharma. So equality and inequality are apparent, and not essential. These differences are because of the diversity in methods, and differences in the stages and phases through which each being is going at any point in time.
Windows
However, multiple windows to Truth describe the eternal, its relation to the universe, and the concept of liberation. These are called Darsanas. They are six and listed as Nyaya, Vaiseshika, Sankhya, Yoga, Purva Mimamsa, and Uttara Mimamsa. They differ in whether there is a "creator-God" different from the eternal, what causes binding and what causes liberation etc, but they all have a common stand, e.g. there is an eternal spirit, indwelling presence (Purusha), who is actually liberated, but appears to be bound by the play of Nature (Prakriti). They are also common in that their goal is liberation. They all advocate righteousness and devotion as a means to liberation, though they suggest different methods. They are also common in that they take Veda as the authority.
Multiple schools describe the relationship between the individual and universal soul, the concept and nature of liberation. These are called schools of spiritual philosophy or Vedanta and they are Advaita, Dwaita, Visistadwaita, etc.
Darsanas and schools of Vedanta are often grouped together, for they address some common subjects. They also commonly believe in concepts like karma and rebirth.
Religion
Hinduism is actually a set of religions, all believing in a common authority - the Veda. Sanatana Dharma can be broadly called the Vedic religion. Each religion has a theology. However they are all not separate theologies, but different interpretations of theologies present in the texts - Veda and the Puranas. They also take relevant sets/portions of rituals, codes prescribed in Sruti and in smritis, and from Agamas.
Smarta is the "religion", where any of the major deities are worshipped and there is no specific leaning to any of them. Literally "smarta" means following "smritis". Then there are six major religions, Saiva, Vaishnava, Sakta, Ganapatya, Saura, and Kaumara, which treat deities Siva, Vishnu, Sakti-the mother Goddess, Ganapathi, Surya, and Kumara Swamy as supreme Godhead respectively.
Purposes
There are four goals of life to fulfill which one should work - Dharma, Artha, Kama, and Moksha. That is Righteousness, generation of wealth/production, the fulfillment of desires, and salvation. The first three are the means to the last.
Paths
There are three paths to salvation. One is devotion (bhakti) where an individual through devotion to God, attains moksha. This is prescribed for the heart-being. Worship is his method; Devotion is the means; bliss and love are the goals.
The second is knowledge (jnana), where one sublimates his lower being through gaining knowledge. This is for the intellect-being. Study/thought is his method; Curiosity his means; Truth the goal.
The third is of works (karma), where one through the fulfillment of responsibilities as an individual and then serving fellow beings, attains moksha. This is for the social being. Service is his method; Selflessness his means; infinity and permanence the goal.
Thus truth-beauty-permanence, the three aspects of the eternal, are the goals of these three paths. They are intermixing and overlapping paths, with each of them leading to and merging with the others.
Scriptures
The root of Sanatana Dharma is the Veda. The Veda is said to be classified before the beginning of Kali Yuga (about 3100 BC according to the traditional version), by Vyasa, into four - Rig, Yajus, Sama, and Atharva. Each Veda has three portions, Samhita (hymns to gods), Brahmana (ritual portion), and Aranyaka (philosophy portion).
There are two major streams of literature, which developed from the Veda and base their authority on it. One of them is the smriti literature, where the entire literature is classified into 18 abodes or mahasthanas. They are the four Vedas, six Vedangas (sastras to understand the Veda, literally the parts of Veda), four Upangas (consisting of dharma viz codes of conduct, etc and logic), and four Upavedas (arts and sciences). Then there are six darsanas or windows to the truth. The second stream is the Agama literature, consisting of Mantra, Yantra, and Tantra.
However these are not mutually exclusive streams, and there are many aspects like underlying spiritual philosophy, mantras, and some procedures for rituals that are common.
Bourne and Definition of Sanatana Dharma
What classifies one as belonging to or outside the fold of Sanatana Dharma, is not as straightforward as answering what classifies one as belonging to or outside the fold of any particular community.
Sanatana Dharma, as it implies, is the eternal law - hence anyone by definition is included in it. It does not classify people as followers and non-followers, believers and non-believers, and so on. Thus by definition, it is universal and all-inclusive. This is because it talks of the natural and universal order or law, and not of the universe as seen by any particular seer.
However, when seen in a social context there do exist many classifications. The people belonging to the civilization of Bharata share a common cultural and social base; they differ in philosophy and a few social aspects. The first such division is Vedic-Unvedic.
Accepting the authority of Veda is the primary criterion, for defining a person as belonging to Veda Mata. And this covers all the branches, such as saute, smarta, Tantric, and so on. However, there are further classifications in this: There are schools that hold Atharva veins as unvedic. Some schools hold some Tantras as unvedic. And so on. However, with all the philosophical and religious differences, they all share one cultural-social base.
The schools that do not hold Veda as a primary authority are called Unvedic. Bauddha and Jaina, though they believe in salvation (having their own versions of Nirvana and Kaivalya respectively) are thus called Unvedic. They do not, however, call the Veda false knowledge. Bauddha and Jaina also share the same civilizational and philosophical base and worldview as the Vedic religion. Hence they are very many parts of the Bharatiya Civilization.
However, this should not be understood in the sense that Veda is to be taken as an "authority". Veda is taken as a Pramana or a source and reference for the validation of knowledge. And a school that does not accept Veda as a brahmana, implies that it does not believe in statements other than those that could be validated through other means of knowledge. There are acceptable pramanas under each school, like pratyaksha, anumana, upamana and so on which could be translated as (perception, logic, comparison, etc.). And the knowledge that could not be validated by these, according to Vedic religion, is to be taken as valid if it finds validation in the Veda. The reasoning given for this by the followers of Vedic religion is that Veda is knowledge of the eternal and contains that knowledge that cannot be validated. So "something that cannot be validated can exist and still be accepted as true" is the premise. And this brahmana was not listed first; in fact, it was listed after pratyaksha and so on, to imply that you do not need the brahmana of Veda for something that can be validated through direct means.
According to Unvedic darsanas like Bauddha, only the knowledge that could be validated by pratyaksha and alumina is to be taken as valid. Thus, the difference is purely at a philosophical level, and not really at a religious level. Therefore we could see there are a lot of similarities in the religious practices of all these schools, such as Tantric and other methods.
However, high-level philosophical differences resulted in social differences, such as accepting the Vedic social order or Varna-Ashrama dharma. Still, they all shared the same cultural-civilizational base. However, it was easy to assimilate them into the social order - for they did not create a new social order, and they were not different socio-cultural systems.
Thus the Vedic-unvedic became a slightly different classification, Astika - Nastika. Two criteria make one an Astika or having astika: believing in Veda's pramanya, and following varna-ashrama dharma. The belief in Veda translating as Astikya borrows sense from the above explanation, of "something that cannot be validated can exist and still be accepted as true".
Accepting God or Iswara has never been a criterion in classifying something as Astika or Vedic or Hindu for that matter. Owing to the differences in approach and diversity in world views, accepting the existence of God has always been a matter of choice and one's philosophy. Diversity in the attribution of supremacy to God, religious practices, and philosophical traits, none of these matter in classifying something as Vedic-Unvedic.
However, Carvaka differs greatly from all other systems including Bauddha and Jaina - it accepts pratyaksha (sense-perception) as the only brahmana. Every other school, Vedic or Unvedic accepts at least two sramanas. And they do not accept akasha as a mahabhuta or a primordial element, which all other schools accept. Akasha is the element that is both the origin of all the other elements and listed as an element (in its unmanifest form). This causes all other differences such as treating the body as self, not accepting rebirth, and so on. In this, Carvaka comes very close to the Western materialistic thought pattern. For this reason, not only did Carvaka have a different philosophy but also an entirely different social style emanating from an entirely alien worldview that is incompatible with all the other schools.
However, all other Bharatiya peoples, including tribal, should be described as sharing the same philosophical base. Either by their practices or by their goals, their origin rests in the same. Any philosophy that came from outside, or developed independently in Bharata and came in contact with Sanatana Dharma, has been assimilated into its comprehensive, all-inclusive system.
However, the ones that are not assimilated are exclusivist ideologies like Abrahamic religions. In a way, they are as philosophically and socially incompatible with Sanatana Dharma as Carvaka was at one time.
So any other school is, technically speaking, belonging to or related to Sanatana Dharma. On the one hand, there are schools like Shanmatas that are part of it, and on the other hand, there are schools like Bauddha that have origin in it but are outgrowths of it.
Knowledge System