What is Shirk ??

Assalaamo Alaikum

What is the significance of Shirk ?

وَإِذْ قَالَ لُقْمَانُ لِابْنِهِ وَهُوَ يَعِظُهُ يَا بُنَيَّ لَا تُشْرِكْ بِاللَّهِ إِنَّ الشِّرْكَ لَظُلْمٌ عَظِيمٌ﴿31:13﴾

(31:13) Remember the time when Luqman was admonishing his son, saying, "My son, join not another as an associate with God: *20 the truth is that joining associates (with God) is a grave iniquity
Is Shirk forgivable ??

إِنَّ اللّهَ لاَ يَغْفِرُ أَن يُشْرَكَ بِهِ وَيَغْفِرُ مَا دُونَ ذَلِكَ لِمَن يَشَاء وَمَن يُشْرِكْ بِاللّهِ فَقَدْ ضَلَّ ضَلاَلاً بَعِيدًا﴿4:116﴾

(4:116) Truly it is only associating others with Allah in His divinity that Allah does not forgive, *144 and forgives anything besides that to whomsoever He wills. Whoever associates others with Allah in His divinity has indeed strayed far away.
Did the Mushriks of Mecca not believe in Allah ??

وَلَئِن سَأَلْتَهُم مَّنْ خَلَقَ السَّمَاوَاتِ وَالْأَرْضَ وَسَخَّرَ الشَّمْسَ وَالْقَمَرَ لَيَقُولُنَّ اللَّهُ فَأَنَّى يُؤْفَكُونَ﴿29:61﴾

(29:61) If you ask them, *100 "Who has created the earth and the heavens and Who has subjected the moon and the sun?" they will surely say, "AIIah." How are they then being deceived?

وَلَئِن سَأَلْتَهُم مَّن نَّزَّلَ مِنَ السَّمَاء مَاء فَأَحْيَا بِهِ الْأَرْضَ مِن بَعْدِ مَوْتِهَا لَيَقُولُنَّ اللَّهُ قُلِ الْحَمْدُ لِلَّهِ بَلْ أَكْثَرُهُمْ لَا يَعْقِلُونَ﴿29:63﴾

(29:63) If you ask them, "Who sent down rainwater from the sky and thereby raised the dead earth back to life?" they will surely say, "Allah!" Say, "Praise be to Allah!" *101 Yet most of them do not use their common sense

وَلَئِن سَأَلْتَهُم مَّنْ خَلَقَهُمْ لَيَقُولُنَّ اللَّهُ فَأَنَّى يُؤْفَكُونَ﴿43:87﴾

(43:87) And if you ask them who created them, they will surely say, "Allah" *69 Whence then are they being deluded?

What was their purpose of Worshiping of Idols ??

وَيَعْبُدُونَ مِن دُونِ اللّهِ مَا لاَ يَضُرُّهُمْ وَلاَ يَنفَعُهُمْ وَيَقُولُونَ هَـؤُلاء شُفَعَاؤُنَا عِندَ اللّهِ

(10:18) They worship, beside Allah, those who can neither harm nor profit them, saying; 'These are our intercessors with Allah.'

أَلَا لِلَّهِ الدِّينُ الْخَالِصُ وَالَّذِينَ اتَّخَذُوا مِنْ دُونِهِ أَوْلِيَاءَ مَا نَعْبُدُهُمْ إِلَّا لِيُقَرِّبُونَا إِلَى اللَّهِ زُلْفَى

(39:3) Beware! Religion is exclusively Allah's right. *4 As for those who have taken other guardians besides Him (and justify their conduct, saying): "We worship them only that they may bring us closer to Allah
Another form of Shirk
Below is from Tafsir Ibne Kasir:

﴿اتَّخَذُواْ أَحْبَـرَهُمْ وَرُهْبَـنَهُمْ أَرْبَاباً مِّن دُونِ اللَّهِ وَالْمَسِيحَ ابْنَ مَرْيَمَ﴾

(They took their rabbis and their monks to be their lords besides Allah, and the Messiah, son of Maryam) ﴿9:31﴾. Imam Ahmad, At-Tirmidhi and Ibn Jarir At-Tabari recorded a Hadith via several chains of narration, from `Adi bin Hatim, may Allah be pleased with him, who became Christian during the time of Jahiliyyah. When the call of the Messenger of Allah reached his area, `Adi ran away to Ash-Sham, and his sister and several of his people were captured. The Messenger of Allah freed his sister and gave her gifts. So she went to her brother and encouraged him to become Muslim and to go to the Messenger of Allah . `Adi, who was one of the chiefs of his people (the tribe of Tai') and whose father, Hatim At-Ta'i, was known for his generosity, went to Al-Madinah. When the people announced his arrival, `Adi went to the Messenger of Allah wearing a silver cross around his neck. The Messenger of Allah recited this Ayah;
﴿اتَّخَذُواْ أَحْبَـرَهُمْ وَرُهْبَـنَهُمْ أَرْبَاباً مِّن دُونِ اللَّهِ﴾
(They took their rabbis and their monks to be their lords besides Allah). `Adi commented, "I said, `They did not worship them.''' The Holy Prophet(PBUH) said

«[B]بَلَى إِنَّهُمْ حَرَّمُوا عَلَيْهِمُ الْحَلَالَ وَأَحَلُّوا لَهُمُ الْحَرَامَ فَاتَّبَعُوهُمْ فَذَلِكَ عِبَادَتُهُمْ إِيَّاهُم»

(Yes they did. They (rabbis and monks) prohibited the allowed for them (Christians and Jews) and allowed the prohibited, and they obeyed them. This is how they worshipped them.)
(Tafsir Ibne Kasir )

Best of luck