In Islam, Shirk is the greatest and most serious sin in the eyes of Allah. It means associating partners with Allah in matters that belong exclusively to Him, including worship, supplication, lordship and His divine attributes.
Here is the evidence from the Qur'an and the authentic hadith.
1. Shirk is the greatest injustice.
Allah, the Most High, says:
Surah Al-Kaf (18:110).
“Say, ‘I’m just a man like you. It was revealed to me, “Your God is one God.” So those who hope to meet our master will do good deeds.”
"Say, 'I'm just a human being like you. It was revealed to me that your God is one God. So whoever hopes for a meeting with his Lord, let him do righteous deeds and not bind anyone in the worship of his Lord. '".
Surah Luqman (31:13).
And when Luqman said to his son while advising him: "O my son, do not associate anything with God. Indeed, polytheism is a great injustice."
"And [remember] when Luqman said to his son while advising him, 'O my son, associate nothing with Allah. Indeed, associating others with him is a great injustice. '".
2. Allah does not forgive shirk if a person dies without repenting.
Surah An-Nisa (4:48).
God does not forgive anyone who associates partners with Him, but He forgives anything but that to whom He wills.
"Indeed, Allah does not forgive partners with Him, but He forgives less than that for whom He wills. ".
The meaning is that those who committed major shirk without repenting will not receive forgiveness from Allah. However, whoever sincerely repents before death will find Allah the Most Forgiving and Merciful.
3. Shirk cancels all actions.
Surah Az-Zumar (39:65).
And it will already be revealed to you and to you: if you connect, your work will be canceled and you will be among the losers.