Shirk (polytheism) is an unforgivable sin

Shirk (Arabic : شرك‎) is the Islamic concept of the sin of polytheism specifically, but in a more general way refers to worshiping other than Allah, associating partners with him, giving his characteristics to others beside him, or not believing in his characteristics. Within Islam , Major Shirk is a forgivable sin if one repents from it while one is alive, but according to Islamic texts, anyone who dies upon this sin will never enter paradise. It is the vice that is opposed to the virtue of tawhid , literally "declaring [that which is] one", often translated into the English term monotheism .

Quran:

You (Alone) we worship, and You (Alone) we ask for help (for each and everything)
Surah Al-Fatihah 1:4


Say (O Muhammad (Peace be upon him)): "He is Allâh, (the) One.{1}.

"Allâh-us-Samad (The Self-Sufficient Master, Whom all creatures need, He neither eats nor drinks){2}.

"He begets not, nor was He begotten;[He does not have any child, parent or spouse]{3}.

"And there is none co-equal or comparable unto Him."{4}.


Surah Al-Ikhlas (112)


[Reason behind revealing this Surah Al-Ikhlas to His Messenger. ]

Ibn-e-khatir wrote in his tafseer ["Ikrimah said, "When the Jews said, `We worship `Uzayr, the son of Allah,' and the Christians said, `We worship the Messiah (`Isa), the son of Allah,' and the Zoroastrians said, `We worship the sun and the moon,' and the idolators said, `We worship idols,' then Allah revealed this Surah Al-Ikhlas to His Messenger. ]

Hadith:

the Prophet (peace and blessings of Allaah be upon him) said: “Du’aa’ is worship.” Narrated by al-Tirmidhi, 2969; classed as saheeh by al-Albaani in Saheeh al-Tirmidhi.

Grave worshiping is also shirk

Hadith :
Narrated 'Aisha and 'Abdullah bin 'Abbas: Abu Huraira:

When the last moment of the life of Allah's Apostle came he started putting his 'Khamisa' on his face and when he felt hot and short of breath he took it off his face and said, "May Allah curse the Jews and Christians for they built the places of worship at the graves of their Prophets." The Prophet was warning (Muslims) of what those had done.
Sahih Bukhari, Book : Volume 1, Book 8, Number 427:


When Umm Salamah told him of a church that she had seen in Ethiopia and the images therein, he said:

“Those people, if a righteous man among them died, they would build a place of worship over his grave. They are the most evil of people before Allaah.”
And it was narrated from Ibn Mas’ood (may Allaah be pleased with him) that


the Prophet (peace and blessings of Allaah be upon him) said: “Among the most evil of people upon whom the Hour will come when they are still alive are those who take graves as places of worship.
” Narrated by Imam Ahmad with a jayyid isnaad. \

Categories of Major Shirk and Minor Shirk

Shirk in Rubūbīyah (Lordship)

This category of Shirk refers to either the belief that others share God's Lordship over creation as His equal or near equal, or to the belief that there exists no Lord over creation at all.

• Shirk by association - This is the shirk concerned with associating 'others' with Allah.
• Shirk by negation - This is Shirk in Rubūbīyah (Lordship)

This category of Shirk refers to either the belief that others share God's Lordship over creation as His equal or near equal, or to the belief that there exists no Lord over creation at all.

Shirk in al-Asma was-Sifat (the names and attributes of Allah)

Shirk in this category includes both the common pagan practice of giving God the attributes of His creation as well as the act of giving created beings God's names and attributes.

• Shirk by humanization - In this aspect of Shirk in al-Asma was-Sifat, God is given the form and qualities of human beings and animals. Due to man's superiority over animals, the human form is more commonly used by idolaters to represent God in creation. Consequently, the image of the Creator is often painted, moulded or carved in the shape of human beings possessing the physical features of those who worship them.
• Shirk by deification - This form of Shirk in al-Asma was-Sifat relates to cases where created beings or things are given or claim God's names or His attributes. For example, it was the practice of the ancient Arabs to worship idols whose names were derived from the names of God. Their main three idols were: al-Lat taken from God's name al-Elah, al-'Uzza taken from al-'Aziz and al-Manat taken from al-Mannan . During the era of Muhammad there was also a man in a region of Arabia called Yamamah, who claimed to be a prophet and took the name Rahman which, in Islam, belongs only to God.

Shirk In al-'Ibadah (Worship)

In this category of Shirk, acts of worship are directed to other than Allah and the reward for worship is sought from the creation instead of the Creator. As in the case of the previous categories, Shirk in al-'Ebadah has two main aspects.

• Ash-Shirk al-Akbar (Major Shirk) - This form of Shirk occurs when any act of worship is directed to other than Allah. It represents the most obvious form of idolatry which the prophets were specifically sent by God to call the masses of mankind away from. Examples of this shirk, asking forgiveness, admittance to paradise and other things that only Allah can provide, from other than Allah.
  

Ash-Shirk al-Asghar (Minor Shirk)

• • Minor Shirk

Mahmud ibn Lubayd reported, "God's messenger said: "The thing I fear for you the most is ash-Shirk al-Asghar (minor shirk)."

The companions asked "Oh! messenger of God, what is minor Shirk?"

He replied "Ar-Riya (showing off), for verily God will say on the Day of Resurrection when people are receiving their rewards, 'Go to those for whom you were showing off in the material world and see if you can find any reward from them."

• Secret Shirk

Mahmud ibn Lubayd also said, "The Prophet came out and announced, 'O people, beware of secret Shirk!'

The people asked, 'O messenger of God, what is secret Shirk?'

He replied, 'When a man gets up to pray and strives to beautify his prayer because people are looking at him; that is secret Shirk."