What are the different sects in Hinduism? And, why are there so many?

For any person who is new to Hinduism, it could be very confusing in the beginning to understand the various sects and their associated philosophies. Due to the apparent complexity, Hinduism is often mistaken for paganism. However, it is not an overstatement to say that Hinduism has the most profound philosophies that have ever existed in human civilization. Based on the interpretation of these philosophies there evolved different sects, and not because of the number of deities. Although the age of this religion has a role to play, the differences in interpretations are endorsed as they match people's different mindsets. 

The differences between these sects are based on the way the following entities and their relationship are defined.

a. Iśwara - The creator
b. Brahman - The fundamental substratum of the universe
c. JIva - The subjects (sentient beings, or individual souls)
d. Prakriti - The world (non-sentient beings/things)
e. Jnana - The right knowledge
f. Mokṣa - Liberation
g. MArga - The path to liberation

Although there are scores of sects each having their interpretation and justification, they can be broadly related to the following few philosophies.

The Philosophy of Duality - Dvaita:

This is the easiest philosophy to understand, which states the universe as Dual in nature. It can also be extended to denote the multiplicity (or pluralism) observed in this universe. Most of the world religions come under the category of Dvaita.

So, what is the duality? The two entities specified in Dvaita are the Creator and His/Her subjects. Due to the innumerable number of the subjects (both living and inert), this philosophy also extends itself to denote the multiplicity nature of the universe. The philosophy revolves around the attributes of the Creator and His/Her relationship with the subjects.

According to Dvaita, Iśwara - the Creator is Omnipotent, Omnipresent, Omniscient, and is the primal cause of the entire creation. In the scriptures, the term Brahman is used to indicate the fundamental substratum of the existence of the Universe. Thus as per Dvaita, Iśwara is Brahman. The closest word that relates to Iśwara with respect to other world religions is the term 'God' (or other equivalents). Here, God is a personal entity and is Saguna, endowed with attributes and qualities. The world we see is the effect of God's creation. It is this God that is Supreme and all other divine beings (demigods), sentient living beings, and inert things are all dependent on this One Universal Being. The sentient subjects or the JIvas are countably infinite and are distinct from one another. Albeit not being synonymous, 'soul' is the term that comes closest to the term JIva. Due to the lack of better words, the terms 'God' and 'soul' will be used in their respective contexts for easier understanding.

Dvaita says the world that we see is as real as it is, and objects of the world are distinct from one another, which seems quite straightforward. However, it is worth mentioning because this stands in contrast to the Advaitic school of thought which will be discussed later. The purpose of the lives of the individual souls is to offer Bhakti (Love/Devotion towards God), through prayers and offering service to God. By treading the path of Dharma (Righteousness) as specified in the scriptures, individual souls establish themselves in the service of God through Karma (Actions that are directly or indirectly associated with the service of God) and Bhakti, leading to Jnana (The Knowledge). The term Jnana in this context means realizing that the individual soul is eternally dependent on God. The individual souls get liberated from the material bondage of the world when they realize all that exists is essentially dependent on the One Universal Supreme God, Iśwara. Mokṣa is attained at the end of a Jnani's lifetime, where the Jnani is taken to the abode of Iśwara and the former engaged in the service of the latter. It is to be noted here that failure to follow the path of Dharma leads to the eternal damnation of souls.

The steps to release from the bondage of the birth-death cycle

As mentioned earlier, Dvaita specifies a Supreme monotheistic God akin to other major religions of the world. In Hinduism itself, there are different sects that can be classified under Dvaita. The most significant difference between these sects is the primary God who is considered as Iśwara. Although there are many such sub-schools, the term Dvaita is predominantly associated with the 13th-century school established by Madhvacharya, in which Lord Vishnu is considered to be the Supreme Being. Some of the other similar philosophies include Shaivam (Lord Shiva),  Gaudiya Vaishnavam (Lord Krishna), ShAktam (Goddess Shakti), and many more.

Metaphor: The Potter and the Pot - A potter makes a pot out of clay. Similar to how the clay pot is real and distinct from the potter, likewise, an equally real universe exists with its own separate essence from that of God. Thus, there are two simultaneous and independent realities that exist.

The Philosophy of Qualified Monism - VishishtAdvaita 

Qualified monism or qualified non-dualism is the school of thought that believes in all diversity subsuming to an underlying unity. Although there are many variants of this philosophy, we will explore two main streams.

Sri Vaishnavam

The main proponent of this philosophy, Ramanujacharya, is credited as an important exponent of this philosophy, with Lord Vishnu as the primary God of worship.

VishistaAdvaita specifies a qualified monism, where it supports the possibility of the JIva sharing the same nature as that of Brahman, and can reach the bliss state of God while retaining their individual identities even after Mokṣa. Here Brahman is nothing but Iśwara with all the qualities and attributes, i.e., Saguna Brahman.

As per this school, all JIvas are distinct as in Dvaita which can never be transcended, and there are different grades in them. Similar to Dvaita, the world is real and distinct. Jnana is knowing this as the truth. After Jnana, the individual treads the path of devotion through Karma and Bhakti. Only by practicing Bhakti, the JIva ultimately attains Mokṣa at the end of one's lifetime breaking away from the cycle of birth and death. Even after Mokṣa, individual souls retain their identity but eternally remain in fellowship with Iśwara.

Shaiva Siddhanta

Shaiva Siddhanta propounds a philosophy where the ultimate and ideal goal of JIva is to become an enlightened soul through Lord Shiva's grace. Although there are some similarities with respect to VishistaAdvaita of Ramanujacharya, there are some differences too. The most obvious one is that in Shaiva Siddhanta, Lord Shiva is the ultimate cause of creation, and is the only Iśwara, as opposed to Lord Vishnu as in Sri Vaishnavism. 

Metaphor: Sun, stars and the sky - In the night sky, we see countless stars. When the sun rises, the brightness of the sun occupies the entire sky, and throughout the day the stars are nowhere to be seen. It is as if the stars have merged into the brightness of the sun. Yet, stars retain their individuality which is evident every night.

The Philosophy of Monism - Advaita:

Fundamentally differing from all other viewpoints, the Advaita philosophy presents itself in sharp contrast with that of Dvaita and Vishishtadvaita. Although the roots of Advaita can be traced to a much earlier time period, the most prominent exponent of this philosophy is considered to be Adi Śankaracharya. Advaita Vedanta is arguably the oldest extant of the sub-schools of Hindu philosophies and is also the most misunderstood one. Although Advaita embraces most of the concepts accepted by Dvaita or VishishtAdvaita, it fundamentally differs from their philosophy. Literally, the term Advaita means 'without a second'. It accepts the concept of Universal Supreme Being, Iśwara, which is Saguna Brahman in accordance with that of Dvaita and VishishtAdvaita. However, it states that there exists an ultimate reality even beyond this, termed Nirguna Brahman, the One beyond any attributes or qualities. The JIva or the individual souls are just projections of this very same reality, and so is the non-sentient world. In other words, this philosophy states that all the souls are in fact, just only One Universal Soul, the Atman, which again is the same as the Universal Supreme Entity, the Brahman, and everything else is just an illusion. Due to this, the philosophy is often referred to as MAyavAda. Below is a crisp summary of this philosophy quoted from Brahmajnanavalimala (verse 20) by Adi Śankaracharya.

ब्रह्म सत्यं जगन्मिथ्या जीवो ब्रह्मैव नापरः ।
Brahma satyam jagat mitya, jIvo Brahmaiva nAparaha |

Meaning: Brahman is the only truth, the world is illusory, and that Brahman and the individual self are non-different.

The word mitya is often misunderstood to mean 'unreal' rather than 'illusory' leading to misinterpretation of the entire philosophy.

According to this, there are no distinct individual souls that exist. It is because of the power of MAya every individual 'thinks' each one is a different being, and that the world is real and distinct. One 'thinks' this way due to avidyA, or the lack of knowledge. MAya (Lit., magic), the illusory power of Iśwara, is the instrument for creation, because of which the world appears to be real, and it does so only so long as one associate with it.

Metaphor: The rope and the snake - Imagine walking alone in a dark wilderness. Suddenly you step on something that is slender, soft, and cold. In that situation, you think you have stepped on a snake. However, in a few moments with the help of ambient light, you realize that you have actually stepped on a rope. For a very brief amount of time, you were fully convinced that you have indeed stepped on a snake. The fear was real. But once you realize that it is not a snake, but only a rope, the fear is now permanently gone. But the rope is still present. Everything around you is still the same. It is just that the knowledge of the rope has destroyed the snake in it. Was the snake real or unreal? You can still step on it and feel it the same way it felt before. But now there is no more fear as the knowledge of the rope has been obtained. This knowledge is Jnana.

Metaphor: Waking from a dream - Dreaming is a very common experience everyone goes through during sleep. One may experience things that are not possible in the waking world. For instance, you may dream of flying like a bird. The physics of the dreamworld may permit a human to fly like a bird. You will be fully convinced of the science behind the flying. There is no question on the logic that is applicable in the waking world as long as you are still dreaming. The moment you wake up, you realize it was all just a dream. You still don't question the physics in the dream world, because it really does not matter. Similarly, so long as one associates oneself with this world, the logic seems perfect. The moment one gains Jnana, there is no question about the reality or logic in the world.

Thus, Advaita provides scope for every individual soul to ascend step-by-step in the path of Dharma, via Karma, Bhakti, and VichAra (questioning), ultimately leading to Jnana. Jnana here means the knowledge of the Self, which is the only thing that is real. The highlight of Advaita is that Mokṣa is nothing but obtaining Jnana, which is possible in one's lifetime itself, unlike in other philosophies where Mokṣa is possible only at the end of one's lifetime. That is, the true liberation from the birth-death cycle is instantaneous with the attaining of Jnana. It is to be noted that there is no scope of JIva merging with Brahman/Iśwara simply because merging is possible only when there is more than one entity. 

A common pitfall here is that one may interpret Advaita in a way that equates the individual soul to God. Even as per Advaita, it is blasphemous to compare JIva with Iśwara. The comparison is invalid, as Iśwara is always associated with the Saguna Brahman that has infinite attributes and qualities, while JIva, the individual soul (associated with the material world) has only an infinitesimally small fraction of it. The non-difference can be realized only by transcending these attributes by the mercy of Iśwara.

Although Advaita popularly refers to the philosophy expounded by Adi Śankaracharya, there are various nuances in this philosophy as well with subtle differences, each forming a separate sub-school on its own.

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In all honesty, it may seem confusing to a beginner to understand these in-depth and most importantly to choose which path to follow. Due to disagreements between these philosophies, it may also seem that none of them are fully satisfactory. With little effort, this confusion would fade away because, at the level of common persons like us, these differences don't matter. It can be inferred that in spite of the differences, all these philosophies do have several commonalities. The table below illustrates these. 

Entities	Dvaita	VishishtAdvaita	Advaita
Iśwara - The creator	Primal cause of creation	Primal cause of creation	Primal cause of creation
Brahman - The fundamental substratum of the universe	Saguna (having infinite attributes)	Saguna (having infinite attributes)	Acknowledges Saguna Brahman. Also specifies Nirguna Brahman beyond all attributes.
JIva - The subjects (sentient beings, or individual souls)	Countably Infinite, real, and distinct. Different from Brahman, which is never transcended. Inferior to the nature of Brahman.	Countably Infinite, real, and distinct. Different from Brahman, which is never transcended. But, shares the same essential nature.	One, and fundamentally same as Brahman.
Prakriti - The world (non-sentient beings/things)	Real and distinct.	Real and distinct.	Illusory (Appears to be real due to MAya).
Jnana – The right Knowledge	Knowing and realizing the above.	Knowing and realizing the above.	Knowing and realizing the above.
Mokṣa - Liberation	Breaking the cycle of birth and death. Achieved at the end of one’s lifetime.	Breaking the cycle of birth and death. Achieved at the end of one’s lifetime.	Breaking the cycle of birth and death. Achieved instantaneously at the moment of Jnana.
MArga - The path to liberation	To practice Dharma via Karma and Bhakti as stated in scriptures.	To practice Dharma via Karma and Bhakti as stated in scriptures.	To practice Dharma via Karma and Bhakti as stated in scriptures. In addition, Vichara (questioning) helps.

To conclude, one must follow the righteous path of Dharma as specified in the scriptures. Fortunately,  there are no contradictions in Dharma accepted by all these philosophies on what is right and what is wrong. In other words, the MArga to attain Mokṣa is the same across all these philosophies. The differences come into the picture only with respect to the highest state achieved by the individual souls. By following the instructions of a proper Guru, one should carry on their duties and allow the grace of the Lord to govern the rest.

Reference:
Deivathin Kural (Voice of God) Volume 4, Compiled by R.Ganapathy, Vanathi Pathippakam