The heinousness of Shirk is evident in the fact that it implies undermining the grandeur of Allah, Exalted be He, by claiming equals to Him, which is sheer misguidance! Allah, Most High, says about the polytheists on the Day of Resurrection when they shall dispute with their idols (interpretation of meaning): "By Allah, we were indeed in manifest error * When we held you as equals to the Lord of the worlds. Quran 26:97-98. Al-Qurtubee said: “The meaning of His saying: When we held you as equals to the Lord of the worlds, is declaring you equals to Allah in worship). [48] See Tafseer Al-Qurtubee 13\116

This undermining of the greatness of Allah, Exalted be He, is the gravest act of injustice. Allah, Most High, says (interpretation of meaning): Indeed, association partners with Him is great injustice. Quran 31:13  Al-Sa’dee said: “The reason for such gravity is that there is no more heinous act than holding a being created from dust as equal to the One All-Powerful Creator, and claiming equality between a created being who cannot dispose of any of the affairs of the worldly life and the One who disposes of all affairs, and equating the imperfect being who is needy in every way to the Perfect Lord who is Rich in every aspect, and between the one who does not confer any blessing on people and the Bestower of all blessings with regard to their religion, worldly life, the Hereafter, hearts, and bodies, and none can ward off evils except Him. Is there a graver and more heinous form of injustice? Is there a graver act of injustice than that committed by the one whom Allah created to worship Him and adhere to Tawheed, but he degrades his honored ‘self’ into the lowest ranks, by worshipping someone or something unworthy of his worship? By doing so, he has wronged himself tremendously. [49] See Tafseer Al-Sa‘dee p. 648

Likewise, Shirk implies thinking ill of Allah, Exalted be He, The Lord of the worlds.
Ibn Al-Qayyim said: “Shirk and Ta’teel are founded on having ill thoughts about Allah … A polytheist may think that Allah, Exalted be He, needs someone else to dispose of the affairs of the worldly life alongside Him ... and this implies undermining the greatness of Allah, who stands in no need of anything or anyone else … He may also think that the Power of Allah is perfect only when coupled with that of a partner, or think that He does not know things until a mediator informs Him, does not bestow His mercy upon His creation unless an intermediary makes Him, or does not grant His slave’s wishes unless an intermediary intercedes with Him in his favor, like a created being would intercede for another created being, and he is obliged to accept his intercession as he himself benefits from that interceder and is in need of him, to avail himself of abundance after scarcity or glory after humiliation. He may also think that Allah does not answer the supplication of His slaves until they ask the intermediary to submit these requests to Him, like kings in the worldly life. In fact, this is the very essence of Shirk by associating created beings with Allah, The Exalted … He may also think that He does not hear their supplications, being far from them until an intermediary communicates them to Him, or thinks that a created being has a right over Allah and therefore he would implore Allah by the right of that created being to fulfill his needs, and he pleas Allah by means of this created being, as people might seek the intercessory assistance of those endeared to high officials and kings to act as intermediaries and help fulfill their needs.” [50] See Ighaathat Al-Lahfaan min Masaayid Al-Shaytaan 1\103
Another manifestation of the heinousness of Shirk is that Allah, The Almighty, described its adherents as impure.

Allah, Most High, says (interpretation of meaning): O you who have believed, indeed the polytheists are impure, so let them not approach Al-Masjid Al-Haraam after this, their final year. Quran 9:28 Ibn Al-Jawzee said: (There are three opinions on the interpretation of ‘impurity’ in this context. First, it could mean that their bodies are ritually impure, similar to the religious ruling on dogs and pigs ... Second, it could mean that they are similar to the ritually impure because they neglect the obligatory Ghusl (i.e., ritual bath) for Janaabah (i.e., post sexual-intercourse impurity), even though their bodies are not actually unclean … Third, it could mean that since it is obligatory on us to avoid them just as we are commanded to avoid all that is declared ritually impure by the Sharee’ah, they are held in the same standing of what is ritually impure in this regard, and this is the opinion held by the vast majority of scholars.” [51] See Tafseer Ibn Al-Jawzee 2\248  Another manifestation of the heinousness of Shirk is that it is impermissible for a Muslim to marry a polytheist.
Allah, Most High, says (interpretation of meaning): And do not marry polytheistic women until they believe … And do not marry polytheistic men to your women until they believe. Quran 2:221

Al-Barkawi (Birgivi) said: “Shirk was the gravest act of injustice and most heinous misdeed, and therefore it is the most hated sin to Allah, The Exalted. Accordingly, He prescribed for it punishments in the worldly life and the Hereafter, like no other sin. He informed us that He does not forgive Shirk and that those who commit it are considered ritually impure and forbade them from approaching His Sacred Mosque. He also declared their animal sacrifices unlawful, forbade Muslims from marrying them, deemed it impermissible for the believers to take them as close friends and allies, and took them as enemies to Him, and to His Angels, Messengers, and Believers. At times of war, He deemed it allowable for Muslims to avail themselves of their wealth (seized as spoils of war), take them and their women as captives (during battle), and this is because Shirk implies undermining the right of Allah, Exalted be He, to Lordship and the greatness of His Divinity, and thinking ill the Lord of the worlds) [52] See: ((Ziyaarat Al-Quboor Al-Shar‘iyyah was Al-Shirkiyyah)) (p.: 26) .

As for the gravity of Shirk, it shall be delineated below:

1- It renders one’s good deeds void and worthless:

Allah, Most High, says (interpretation of meaning): And it was already revealed to you and to those before you that if you should associate anything with Allah, your work would surely become worthless, and you would surely be among the losers." Quran 39:65 Al-Sa’dee said: “It was revealed to all the Prophets of Allah that Shirk renders all one’s good deeds void and worthless as He Says in Al-Maa’idah chapter (no.5) after referring to a number of His Prophets and Messengers. He Says (interpretation of meaning): That is the guidance of Allah by which He guides whomever He wills of His slaves. But if they had associated others with Allah, then worthless for them would be whatever they were doing. Quran 6:88 He also Says (interpretation of meaning): … and you would surely be among the losers." Quran 39:65 This means that such person would lose his religion and Hereafter; Shirk causes one’s good deeds to become void and worthless and endangers him to the prescribed punishment and torment [53] See Tafseer Al-Sa‘dee p. 729 .

2- If the doer dies while adhering to Shirk, he will not be forgiven. Allah, Most High, says (interpretation of meaning): Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills. Quran 4:4 Ibn Aboo Al-Khayr said: “The meaning of the ayah is that Allah does not forgive those who associate partners with Him and die while adhering to Shirk, and He forgives the sins that are less grave than Shirk for whomever He wills from among the people of Tawheed). [54] See Al-Intisaar Fee Al-Radd ‘ala Al-Mutazilah Al-Ashraar 3\669  Al-Sa’dee said: (This noble ayah refers to the ones who do not repent of Shirk. As for the repentant, Allah forgives their sin (Shirk) and the less grave ones as well.) [55] See Tafseer Al-Sa‘dee p. 182

3- He who dies while adhering to Shirk will abide in Hellfire perpetually. Allah, Most High, says (interpretation of meaning): Indeed, he who associates others with Allah – Allah has forbidden him Paradise, and his refuge is Hellfire. And there are not for the wrongdoers any helpers. Quran 5:72 Al-Sa’dee said: (The ayah means that whoever commits Shirk, associating a created being, whether Jesus or anyone else, with Allah, He will deny him access to Paradise and render Hellfire his refuge, because he held the created beings equal to the Creator, and devoted to them sincere worship, which must be exclusively devoted to Him, and therefore deserved to be perpetuated in Hellfire.” [56] See Tafseer Al-Sa‘dee p. 240
Allah, Most High, says (interpretation of meaning): Indeed, they who disbelieved among the People of the Scripture and the polytheists will be in the fire of Hell, abiding eternally therein. Those are the worst of creatures. Quran 98:6

It was narrated on the authority of Jaabir (may Allah be pleased with him) that a man came to the Prophet ? and asked him, “O Messenger of Allah, what are the two imperatives which lead to Paradise or Hellfire.” He ? replied, "He who dies without associating anything with Allah will enter Paradise, and he who dies associating partners with Allah will enter Hellfire.” [57] Narrated by Muslim no. 93
Moreover, the gravity of Shirk is further highlighted by the following verities:

1- The covertness of some of its forms and the vulnerability of some righteous and knowledgeable people to it. It was reported on the authority of Aboo Waaqid Al-Laythi (may Allah be pleased with him) that when the Messenger of Allah ? went out to Hunayn, he passed a tree that the idolaters called Thaat Anwaat upon which they hung their weapons. They said: “O Messenger of Allah! Make Thaat Anwaat for us as they have one!” The Prophet ? said: “Subhan Allah! This is like what the people of Prophet Moosaa (Moses) said: O Moses, make for us a god just as they have gods. Quran 7:138 By the One in Whose hand is my soul! You shall follow the way of those who were before you." [58] It was narrated by Al-Tirmithi (2180) with this wording, and Ahmad (21897) on the authority of Aboo Waaqid Al-Laythi (may Allah be pleased with him). Al-Tirmithi said: “It is Hasan Saheeh.” It was classified as Saheeh by Al-Albaanee in “Saheeh Sunan Al-Tirmithi” (2180). Ibn Baaz stated that its chain of narration is authentic in “Majmoo’ Al-Fataawa” (3/337), and Shu‘ayb Al-Arnaa’oot as well, and stated that it met the conditions stipulated by Bukhaaree and Muslim, in his revision of “Musnad Ahmad” (21897). Ibn Al-Qayyim stated that it is authentically attributable to the Prophet ? in ((Ighaathat Al-Lahfaan)) (2/418). The Hadeeth was narrated with the following wording: ((You shall follow the way of those who were before you)). It was narrated by Al-Bukhaaree (7320) with this wording, and Muslim (2669) on the authority of Aboo Sa‘eed Al-Khudree (may Allah be pleased with him).
Muhammad ibn Abd Al-Wahhaab said: (This story indicates that a Muslim, even a scholar, may fall into some forms of Shirk of which he may be unaware, and therefore it is deduced that learning (about Shirk) and caution are required (of all people).” [59] See Kashf Al-Subuhaat p. 45

2- The fact that Prophet Ibraaheem (peace be upon him) and Muhammad ? sought refuge with Allah from it. Prophet Ibraaheem (peace be upon him) supplicated (as recorded in the Quran): My Lord, make this city Makkah secure and keep me and my sons away from worshipping idols. Quran 14:35 Ibraaheem Al-Taymi said: (Who could possibly be safe from such peril after learning the words of Prophet Ibraaheem, “Keep me and my sons away from worshipping idols”? [60] See Tafseer Al-Quraan Al-‘Atheem by Ibn Aboo Haatim 7\2249  It was narrated on the authority of Anas (may Allah be pleased with him) that the Prophet ? used to supplicate Allah, saying: “O Allah, I seek refuge with You from helplessness and laziness, miserliness and aging, cruelty and negligence, disgrace and poverty, Shirk and hypocrisy, and from ostentation and showing off.” [61] It was cited by Ibn Hibbaan (1023), Al-Tabarani in Al-Mu‘jam Al-Sagheer (316), and Al-Haakim (1944). It was classified as Saheeh (authentic) by Ibn Hibbaan (no. 1023), Al-Haakim who stated that it met the conditions stipulated by Bukhaaree and Muslim, Al-Albaanee in Saheeh Al-Jaami‘ (1285), Al-Waadi‘i in “Al-Saheeh Al-Musnad” (35), and its chain of narrated was classified as authentic by Shu‘ayb Al-Arnaa’oot in his revision of “Saheeh Ibn Hibbaan” (1023).