Shirk means that you associate others with Allah, whereas He is The One Who created you; and you worship other than Him, whether it is a stone, a tree, moon, or a prophet, or a scholar, or a genie (jinn), or an animal, or an angel, or the like.
Allah the Holy and Exalted decreed, which can be translated as,
“Surely Allah does not forgive that a partner be ascribed to Him, although He forgives any other sins for whomever He wills. He who associates anyone with Allah in His Divinity has indeed forged a mighty lie and committed an awesome sin.” (QS. An-Nisa/The Women: 48)
Allah also decreed, which can be translated as,
“And surely they disbelieved when they said: “Christ, the son of Mary, is indeed God”; whereas Christ had said: “Children of Israel! Serve Allah, Who is your Lord and my Lord.” Allah has forbidden Paradise to those who associate anything with Him in His Divinity and their refuge shall be the Fire. No one will be able to help such wrong-doers.” (QS. Al-Maidah/The Table Spread: 72)
And He also decreed, which can be translated as,
“And call to mind when Luqman said to his son while exhorting him: My son, do not associate others with Allah in His Divinity. Surely, associating others with Allah in His Divinity is a mighty wrong.” (QS. Luqman: 13)
Also, verses regarding this major sin are numerous.
Hence, whoever associated partner to Allah, and he died as an infidel (polytheist), he is certainly among the dwellers of Hell. As one who believed in Allah, and he died as a believer, will surely be among the dwellers of Paradise, although it is possible that he’ll be tortured first in Hell (due to his past sins, beside shirk; meaning that he committed many sins but not shirk).
And the Prophet -peace and prayer of Allah be upon him- said,
“Would you like me to tell you what is the most serious of sins? That is: shirk to Allah…..” (Taken from a hadith)
The Prophet -peace and prayer of Allah be upon him- also said,
“Avoid the seven destroyers.” It was said: “What are they, O Messenger of Allah?” He said: “Associating others with Allah (Shirk)….”
And the Prophet -peace and prayer of Allah be upon him- also said, “Kill whoever change his religion.” Reference: “76 Major Sins that are Considered As Usual Things.” By Al Imam Adz Dzahabi. Studied by Muhyiddin Misti. Publisher: Darul Haq 3rd edition, 2011/1432 H.
Article of www.whatisquran.com
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