Why is the Heart Sutra so important in Mahayana Buddhism?

The Sutra is rather famous instead of important because it was mentioned in one of the most popular Chinese Novel and Movie making, The Journey to the West.

In the centuries following the historical Xuanzang, an extended tradition of literature fictionalizing the life of Xuanzang and glorifying his special relationship with the Heart Sūtra arose, of particular note being the Journey to the West

https://en.wikipedia.org/wiki/Heart_Sutra

Several terms have been used to refer to the novels and various subgroupings of them. Romance of the Three Kingdoms, Journey to the West, Water Margin, and Dream of the Red Chamber are most commonly grouped as the Four Great Classic Novels (Chinese: 四大名著; pinyin: sì dà míngzhù; lit.: 'four great masterpieces').[6][7] Another term used is Classical Novels (simplified Chinese: 古典小说; traditional Chinese: 古典小說; pinyin: gǔdiǎn xiǎoshuō).[8][9] Prior to the composition of Unofficial History of the Scholars and the Dream of the Red Chamber, the earlier four began to be referred to as the Four Great Masterworks (四大奇书; 四大奇書; sì dà qíshū; 'four extraordinary books').[10]

https://en.wikipedia.org/wiki/Classic_Chinese_Novels

In my opinion, the sutra has no hidden or mysterious purpose. Instead, in my opinion, the sutra is a product of 'bright delusion' or 'white darkness'.

The sun provides light to enable people to see. But if people look directly at the sun, their eyes become 'blinded by the light'. Similarly, certain states of samadhi or concentration give rise to 'bright delusion' or 'white darkness'

The temporary liberation of the illusion of 'non-conceptuality' gives rise to the conception that non-conception is liberation; similar to how, before his enlightenment, teachers of the Buddha regarded the sphere of 'nothingness' as Nibbana.

In my opinion, there is no such thing as non-perception or non-conception (unless in nirodha sampatti, which is a state of samadhi induced unconsciousness). If something can be experienced & described, it has been perceived & conceived. In MN 43, it is said: "wisdom & consciousness are co-joined". Yet the Heart Sutra declares there is 'no cognition'. If there was 'no cognition', such 'non-cognition' must be cognised for there to be the cognition that there is 'no cognition'.

The Pali version of Buddha defines sunnata as 'empty of self'. Yet Nargujana appears to have defined shunyata as 'empty of inherent existence'. Nargujana regarded Nirvana & samsara as the same, thus appearing to negate both.

Therefore, in my opinion, the sutra has no hidden purpose. Instead, the sutra is an idiosyncratic conception about (a 'non-existent') enlightenment since it appears to conform with the conceptualizations (philosophy) of Nargajuna.

Given that all the sutras are skillful means, one would imagine the heart sutra is popular because it works, or at least gives the impression of doing so.

I like it.

Buddhist philosophy isn't, and doesn't need to be, an analytic argument, because it is there to be realised in meditation, not (just) studied and unpacked.

So instead of trying to unpack it, why not ask what the rest of the canon has that it lacks? Given that it mentions, as you say, the basic terms of buddhism (emptiness, the noble truths), perhaps it can encapsulate those teachings, by denying them.

I think the key phrases are

There is No Wisdom, and There is No Attainment Whatsoever... The Buddhas... Have Attained Supreme Enlightenment

Which suggests (to me) that enlightenment is void, form.

Whether or not that means that it is anything beside (fearless) verbal quiescence, I really don't know.

First I'm not expert, this is my personal opinion based on what I have read, and things that I been told by people that study the Dharma, the field in enormous and require lots of study, reader is advice that I could be wrong.I will add a short list or my resources at the end.

I believe the reason of its importance is because it includes the complete teachings of the Vajrayana Buddhisms in it. Emptiness and Wisdom. As well give a concrete explanation based on logic and prove that everything is empty of the essential nature.

I pass to explain why I think this, but first a little bit of context.

Where the Heart Sutra comes from?

The short answer is that it comes from the Buda, or at least the Buddha is giving the teaching with out giving it. (Emptiness) Since, start with the phrase “ Thus i heard on one occasion...”. Is believe that confirm authenticity as was used to prove this since earlier text. Also there is Tibetan lineages with proven unbroken lineage that have the Heart Sutra as teaching since their origins (some old as 1000 years old, Sakya linage). Beside commonly believed to be an oral tradition from the Buddhas times, hard copies have been discovered, some old as around 1400 year in what we known today as China. The work stand by itself, as its use logic for the demonstration of the of emptiness, although we still need faith to believe that comes from the Buddha's time.

To understand better, probably is necessary to have certain understanding of the “frame of work”, which is the human body. The definition of a human body was a bit different as is today, and the text refers to this when mention: “...who wishes to practice the profound perfection of discriminating insight should discern in this way: They should correctly view those five aggregates also as empty of essential nature.”

The five aggregates are: Five senses Organs, Feelings, Perception, Conditioning factors, consciousness.

They are a “part” of the enumeration of the phenomena and Enumeration of phenomena is kind of the definition of the human experience, or a human, but keep in mind that this is a way to break down what can not be broken down.

  Form
    |---Eye and the object to see
    |---Ear and the object to hear
    |---Nose, Odor
    |---Tongue, Flavor
    |---Body, Object of touch
    |---It also considers the imperceptible
 Feelings
    |--- Positive
    |--- Negative
    |--- Neutral
 Perception
    |
 Conditioning Factors
    |
 Consciousness
    
    

The reason that Avalokiteshvara (Chanrezi) is a figure in the Heart Sutra is because while practicing compassion, we see others as our-self or our-self as others and that is very helpful when we are trying to understand emptiness.

Also emptiness is represented as a illusory lady in the same text she is called prajnaparamita.

Please forgive any typo and this is just my personal opinion. Gracias!

Bibliography

1. Abhidharmakosabhasyam volume I English version by Leo M. Pruden Asian Humanity press 1988-1990

2. A praice of the twelve exemplary deeds of Buddha-The Heart Sutra-A non- sectarian prayer. Sakya Monastery of Tibetan Buddhism English Translation Cyrus Stearns edited by Jeffrey Schoening Sakya Monastery 2000

3. https://84000.co/insights-from-a-teacher-an-interview-with-dilgo-yangsi-khyentse-rinpoche

Perhaps

its terse nature and convenience for chanting (quote from your question)

is the exact reason that it is popular. I believe all the Prajnaparamita sutras point to the same thing in the same kind of way. Why wade through the perfection of wisdom in 8,000, 25,000 or even 100,000 lines when you can pop the Heart sutra in your pocket and off you go. It's why I know it and haven't read any of the others.

So I posed this question to my teacher at the temple yesterday, and the answer floored me!

I asked her what is the meaning of "No suffering, origination, cessation of suffering, path" in the Heart Sutra, and if it was some kind of crazy wisdom. She replied:

"No, this is an understanding of emptiness, that is an understanding that things are caused through causes and conditions (dependent origination). Like a cup of coffee implies the existence of coffee beans, although coffee beans by themselves do not necessarily result in coffee because you require other conditions such as the sun or farmers. When the seed is removed, the fruit does not exist. When a person attained peace and happiness and resolved their suffering, the suffering they had is in the past and no longer exist in the present. Because of the awareness of this, a Boddhisattva can work to liberate beings with no fear of suffering in Samsara because he knows that ultimately suffering is impermanent."

And as the sutra goes:

"He passes far beyond all confused imagination and reaches Ultimate Nirvana"

Mind blown. I can understand why Xuanzang put so much emphasis on this sutra based on his difficult experiences on his journey now.

The Heart Sutra is of utmost importance in Mahayana because it declares the key realization that makes Mahayana distinct from a superficial "Sutra-Yana":

That in the ultimate sense all concepts are arbitrary, including even the concepts comprising Buddha-Dharma, that ontological reality behind concepts is by itself free from any discriminations, that freedom of mind from the clashes induced by conceptual discriminations is the very freedom from suffering that Buddha taught.

Heart Sutra is the Mahayana's Declaration of Independence in that it proclaims the heart of the teaching, the transcendental wisdom, the wisdom that goes to the end of the realm of thought, the limits of conceptualization.

It's not written for a shocking effect. The reason it emphasizes emptiness of even the most fundamental Buddhist concepts is to drive home the idea that everything is just a mental overlay, and if that's removed then behind it we find no dirt and no need for cleansing, no entrapment and no need for liberation. Our only problem is entanglement in the illusion that these things are real and important.

All past, present, and future Buddhas attain Buddhahood through their realization of this freedom from conceptual discrimination. This realization is not something external to the Canon but is the real meaning of the liberation as taught by the Buddha in the Canon. Every progression of Jhana states described in the Canon culminates in the very same realization. The parable of the raft, the concept of tathata, the numerous Canonical descriptions of Nibbana - all refer to the same realization.

This proclamation is the heart of Mahayana and that's what makes this Sutra the crown jewel of all the teaching.

Maybe because Clinging/Upadana is such a powerful destructive force, even advanced practitioners still have a tough time dealing with. So the non-clinging theme reflected through insight into Emptiness/Sunyata throughout the sutra would be a great way to remind us about it. That's why in many Mahayana temples this sutra is usually recited at the end of any session, after other sutras have been recited, reminding the practitioner of the Clinging elephant in the room.

Anyway, don't worry too much about why it's the most popular or most important. It's a pretty common stock phrase in Mahayana to say this or that sutra is the most popular or most important. You'll see the same thing being said in the Lotus Sutra, the Amitabha Sutras series, etc. After all, the spirit of the sutra doesn't seem to deviate from the Buddha's teaching about the 3 characteristics inherent in all conditioned dhammas: impermanence/anicca, unsatisfactory/dukkha, and non-self/anatta.

The sutra simply reminds us that it indeed holds true for all "conditioned dhammas", including the Five Aggregates, the Twelve Ayatanas, the Eighteen Dhatus, the Twelve Nidanas, and even the Four Noble Truths!