"And I (Allah) created not the jinns and humans except they should worship Me (Alone)." [51:56]

Here we see the purpose of creation- to worship Allah (azza wajal) alone. On the same note Allah (azza wajal) commands us:

"And verily, We have sent among every community a Messenger [proclaiming]: "Worship Allah [Alone] and avoid the Taaghoot. Then of them were some whom Allah guided, and of them were some upon whom the straying was justified. So travel through the land and see what was the end of those who denied [the truth]" (Qur'an 16:36)

Here we see that Allah (azza wajal) sent every people a Messenger with one message which is to worship Allah (azza wajal) alone and one command to avoid Taghoot.

It is not enough to worship Allah (azza wajal) alone, but we MUST also avoid taghoot. Taghoot, in its linguistic definition, refers to that which goes over and beyond boundaries. In a shari'a definition, Taghoot means anything that is worshipped besides Allah (azza wajal) (ie. leaders, sorcerers, a sheikh, communism, capitalism, etc.)

This ayah is very important to understand because it is very much misunderstood. Many believe the prophets came to tell them to believe in Allah (azza wajal) , that there is only one god, and it is enough to do good actions. But this is false because people already believed in Allah (azza wajal) when the Prophets were sent. The problem was that they associated partners with Him, and this had to be eliminated.

From this ayah we also must understand that La Ilaha ilallah is not affirmed until there is bara'a (enmity, hatred, avoidance of that thing) to the taghoot. That taghoot can be the general taghoot or specific tawagheet. It is important to note that if anyone places anything such as secular knowledge above Allah's words, and makes it their source of standard for truth and falsehood, that secular knowledge becomes a form of taghoot.

As Allah (azza wajal) says:
(This is the) Book (the Qur'an) sent down unto you (O Muhammad), so let not your breast be narrow therefrom, that you warn thereby, and a reminder unto the believers. [Say (O Muhammad) to these idolaters (pagan Arabs) of your folk:] Follow what has been sent down unto you from your Lord (the Qur'an and Prophet Muhammad's Sunnah), and follow not any Auliya' (protectors and helpers, etc. who order you to associate partners in worship with Allah), besides Him (Allah). Little do you remember! 7:2-3

In the same Surah Allah (azza wajal) warns us not to be of those who take Allah's words with a grain of salt while believing wholeheartedly in the proofs from the mundane: "Have you seen those (hyprocrites) who claim that they believe in that which has been sent down to you, and that which was sent down before you, and they wish to go for judgment (in their disputes) to the Taghut while they have been ordered to reject them. But Shaitan (Satan) wishes to lead them far astray. And when it is a said to them: "Come to what Allah has sent down and to the Messenger (Muhammad)," you (Muhammad) see the hypocrites turn away from you (Muhammad) with aversion.

Having said this, we learn that we take the understanding of the Qur'an and the Sunnah according to how the Prophet (sallallahu alayhi wa sallam) understood it, taught it and described it. The ones closest to the Prophet (sallallahu alayhi wa sallam) were ofcourse his companions. His companions then taught their students after the Prophet's death. Then these students taught their students and so on, until this knowledge has become preserved to the scholars of Ahlus Sunnah wal Jama'ah today. Basically, the science of studying Islam, its rulings, and what you have chosen to call isnadaat are not from our own conjecture but from knowledge that has been passed from the time of the Prophet (sallallahu alayhi wa sallam) to the scholars who are the heirs of the prophets.

Last, before we finally address the issue of hijab, it must be noted that there is no separation between what you describe as al-thahir and what is essential in our deen. If we make the mistake of splitting our deen into two matters (what is essential and what is not) then who is to say what is essential and what is not essential in our deen? And what is the proof for each one? By pitting the issue of hijab against "what is supposedly essential" is posing a false dilemma. It is not an issue of either or. All of Islam is one.

For instance, hijab and prayer are important because because everything we do as Muslims comes from the Shari'ah of Allah (azza wajal) . So one cannot say that there is too much emphasis on hijab and not enough on prayer. Both must be observed as they both are observances of Allah's commands. We are incomplete without either one of them. Meaning, if the internal is sound then the outward should be sound as well.

Narrated 'Umar bin Al-Khattab: "People were judged by the revealing of a Divine Inspiration during the lifetime of Allah's Apostle but now there is no longer any more (new revelation). Now we judge you by the deeds you practice publicly, so we will trust and favor the one who does good deeds in front of us, and we will not call him to account about what he is really doing in secret, for Allah will judge him for that; but we will not trust or believe the one who presents to us with an evil deed even if he claims that his intentions were good." [Sahih Bukhari 3 / 809]

The ruling on Hijab with its proofs:
Requirement 1: How far do we cover?
The dress worn in public must cover the entire body except what has been specifically excluded, based upon the following proofs:
1] Allah (azza wajal) says "And tell the believing women to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts, etc.) and not to show off their adornment except only that which is apparent, and to draw their veils all over Juyubihinna and not to reveal their adornment except to their husbands, their fathers, their husband's fathers, their sons, their husband's sons, their brothers or their brother's sons, or their sister's sons, or their (Muslim) women (i.e. their sisters in Islâm), or the (female) slaves whom their right hands possess, or old male servants who lack vigour, or small children who have no sense of the shame of sex. And let them not stamp their feet so as to reveal what they hide of their adornment. And all of you beg Allâh to forgive you all, O believers, that you may be successful. (An-Nur 24:31)
The issue among Islamic scholars of today and the past have never been the issue of defining Juyubihinnah. The majority of the Muslim scholars such as Ibn Abbas (radhi Allahu anhu), Qatadah, Ibn Katheer, Imam Qurtubi, Ibn Hajar al Asqalani, Imam ash-Shawkani, at-Tabari, Imam adh-dhahabi and so on all define the starting point of Juyubihinna as starting from the head not from the bosom. This is evident by how the female companions observed the Hijab, especially the wives of the Prophet (sallallahu alayhi wa sallam) who are examples for the whole Ummah.

The word Khumur refers to a cloth which covers the head hair neck and bosom. The Mufasir al Qurtubi explains:
"Women in the past used to cover their heads with al-khimar, throwing its ends over their backs. This left the neck and the upper part of the chest bare, in the manner of the Christians. Then Allah commanded them to cover those parts with the khimar."
This ayah also lists in detail those with whom a woman is permitted to be without hijab.

2] Allah (azza wajal) says "O Prophet! Tell your wives and your daughters and the women of the believers to draw their cloaks (veils) all over their bodies. That will be better, that they should be known (as free respectable women) so as not to be annoyed. And Allâh is Ever OftForgiving, Most Merciful. (Al-Ahzab 33:59)
The word zeenah above means adornment and includes both that which Allah (azza wajal) has adorned them (i.e. the woman's natural/and or physical beauty) and that with which they adorn themselves (i.e. jewelry, eye shadow, attractive clothing, hand dye, etc.).

Among the authentic hadith that adds to this point is the following:
Ibn Umar reported that Rasulullah (sallallahu alayhi wa sallam) said "On the day of resurrection, Allah will not look at the man who trails his garment along out of pride." Umm Salamah then asked, "What should women do with the hems of their garments?" He replied, "Let them lower them a handspan." She said, "Their feet would be exposed!" He then said, "Let them lower their forearm's length but no more." [reported by at-tabarani Sahih]

This ayah orders Muslim women to draw their cloaks over their bodies when they go out. Abu Dawud related that Aisha said "After this ayah was revealed, the women of al Ansar appeared like crows (because of the color and shape of the cloaks they wore). Hence, an outer garment or cloak must be worn by a Muslim woman whenever she goes out in public or if she is in the presence of strangers.

Requirement 2: How thick should it be?
The garment should not be see-through so as not to display the skin color and form of the body under it. Transparent clothing does not constitute a proper hijab. The following hadith is the proof for this:

Rasulullah (sallallahu alayhi wa sallam) said, "There will be in the last of my Ummah, scantily dressed women, the hair on the top of their heads like a camel's hump. Curse them, for verily they are cursed. In another version he said "..scantily dressed women, who go astray and make others go astray; they will not enter al-Jannah nor smell its fragrance, although it can be smelled from afar." [at-tabarani and sahih Muslim]

"Scantily dressed women" are those who wear clothing which reveals more than it conceals, thereby increasing her attractiveness while opening the path to a host of evils.

Requirement 3: How Loose should it be?
The clothing must not be so tight-fitting as to show the shape and size of the woman's body. The following hadith is the proof.
Usamah Ibn Zayd said, "Rasulullah (sallallahu alayhi wa sallam) gave me a gift of thick Coptic cloth he had received as a gift from Dahiah al Kalbee, and so I gave it to my wife. Thereafter an-Nabee (sallallahu alayhi wa sallam) asked me, 'Why didn't you wear the Coptic cloth?' I replied, 'I gave it to my wife.' an-Nabee (sallallahu alayhi wa sallam) then said, 'Tell her to wear a thick gown under it for I fear that it may describe the size of her limbs.' [narrated by Ahmad, al Baihaqi, and al Hakim]

Requirement 4: What is the color and appearance?
Allah (azza wajal) commands the believing women thus "And stay in your houses, and do not display yourselves like that of the times of ignorance, and perform As-Salât (IqamâtasSalât), and give Zakât and obey Allâh and His Messenger. Allâh wishes only to remove ArRijs (evil deeds and sins, etc.) from you, O members of the family (of the Prophet SAW), and to purify you with a thorough purification. (Al-Ahzab 33:33)

Allah (azza wajal) has commanded women not to display both their natural and acquired beauty. A garment cannot be a thing which enhances her beauty and draw attention to her. Thus, the garment cannot be of bright colors, have Bold designs, or be made of shiny and reflective material. The word above "tabarruj" means not only to display oneself but also to make oneself attractive in public.

Imam adh-dhahabi says in Kitab al Kaba'ir (the book of major sins):"Amongst the deeds which a woman is cursed for are displaying the adornments she wears, wearing perfume when she goes out, and wearing colorful clothes…" Hence, the Muslim woman is encouraged to wear muted, somber colors, and is ordered to avoid bright designs, patterns and colors.

Requirement 5: Should It be different from Men's clothing?
The clothing of a Muslim woman must not resemble the clothing of men. The following are proofs:
Abu Hurairah said Rasulullah (sallallahu alayhi wa sallam) cursed the man who wears women's clothes and the woman who wears men's clothes. [abu Dawood and Ibn Majah-- sahih]

Abdullah Ibn Umar said he heard Rasulullah (sallallahu alayhi wa sallam) say the man who resembles a woman and the woman who resembles a man is not of us [i.e. not of the believers]. [Ahmad and at-tabarani-- sahih]

Requirement 6: Can we Resemble the Dressing of the Disbelievers?
A Muslim woman's clothing must not resemble the clothing of disbelievers. This is a general ruling of the Shari'ah of Allah which includes not on only dress but also manners, customs, religious practices and festivities, transactions, etc. Divergence with disbelievers is a standard that was established by the first generation of Islam. The following is proof for this position:
Abdullah Ibn 'Amr ibn al-As said Rasulullah (sallallahu alayhi wa sallam) saw me wearing to saffron- colored garments, so he said Indeed these are the clothes of kuffar [unbelievers] so do not wear them. [Sahih Muslim]

Abdullah ibn Umar said Rasulullah (sallallahu alayhi wa sallam) stated whoever resembles a people is one of them. [abu Dawood]
Abu Musa al Ashari related that Umar told him "I shall not honor those whom Allah has dishonored, nor esteem those He has humbled, nor bring close those whom he has kept afar." [Ahmad]

Requirement 7: Can we be excessive in our dress?
The Muslim woman's dress must not be an expression of vanity or as a status symbol, nor must it be excessively ragged so as to gain admiration and fame for being humble.

Ibn Umar reported that Rasulullah (sallallahu alayhi wa sallam) said "Whoever dresses for ostentation in this world, Allah will dress that person in a dress of humiliation on the day of resurrection, and then set on fire." [abu dawood]