And most of them believe not in Allah except that they attribute partners unto Him.
Ibn Abbas commented,
"They have a part of faith, for when they are asked, `Who created the heavens? Who created the earth? Who created the mountains?' They say, `Allah did.' Yet, they associate others with Him in worship."
Similar is said by Mujahid, Ata, Ikrimah, Ash-Sha`bi, Qatadah, Ad-Dahhak and Abdur-Rahman bin Zayd bin Aslam.
In the Sahih, it is recorded that;
during the Hajj season, the idolators used to say in their Talbiyah;"Here we rush to Your service. You have no partners with You, except a partner with You whom You own but he owns not!"
Allah said in another Ayah,
إِنَّ الشِّرْكَ لَظُلْمٌ عَظِيمٌ
Verily, joining others in worship with Allah is a great Zulm (wrong) indeed. (31;13)
This indeed is the greatest type of Shirk, associating others with Allah in worship.
It is recorded in the Two Sahihs that Abdullah bin Mas`ud said,
"I said, `O Allah's Messenger! What is the greatest sin?'
He said,
أَنْ تَجْعَلَ للهِ نِدًّا وَهُوَ خَلَقَك
That you call a rival to Allah while He alone created you."
Al-Hasan Al-Basri commented on Allah's statement,
وَمَا يُوْمِنُ أَكْثَرُهُمْ بِاللّهِ إِلاَّ وَهُم مُّشْرِكُونَ
(And most of them believe not in Allah except that they attribute partners unto Him),
"This is the hypocrite; if he performs good deeds, he does so to show off with the people, and he is an idolator while doing this."
Al-Hasan was referring to Allah's statement,
إِنَّ الْمُنَـفِقِينَ يُخَـدِعُونَ اللَّهَ وَهُوَ خَادِعُهُمْ وَإِذَا قَامُواْ إِلَى الصَّلَوةِ قَامُواْ كُسَالَى يُرَاءُونَ النَّاسَ وَلَا يَذْكُرُونَ اللَّهَ إِلاَّ قَلِيلً
Verily, the hypocrites seek to deceive Allah, but it is He Who deceives them. And when they stand up for As-Salah, they stand with laziness and to be seen of men, and they do not remember Allah but little. (4;142)
There is another type of hidden Shirk that most people are unaware of.
Hammad bin Salamah narrated that Asim bin Abi An-Najud said that Urwah said,
"Hudhayfah visited an ill man and saw a rope tied around his arm, so he ripped it off while reciting,
وَمَا يُوْمِنُ أَكْثَرُهُمْ بِاللّهِ إِلاَّ وَهُم مُّشْرِكُونَ
(And most of them believe not in Allah except that they attribute partners unto Him).
In a Hadith, from Ibn Umar collected by At-Tirmidhi who said it was Hasan, the Prophet said,
مَنْ حَلَفَ بِغَيْرِ اللهِ فَقَدْ أَشْرَك
He who swears by other than Allah, commits Shirk.
Imam Ahmad, Abu Dawud and other scholars of Hadith narrated that Abdullah bin Mas`ud said that the Messenger of Allah said,
إِنَّ الرُّقَى وَالتَّمَايِمَ وَالتِّوَلَةَ شِرْك
Verily, Ar-Ruqa, At-Tama'im and At-Tiwalah are all acts of Shirk.
In another narration collected by Ahmad and Abu Dawud, the Prophet said,
الطِّيَرَةُ شِرْكٌ وَمَا مِنَّا إِلاَّ وَلَكِنَّ اللهَ يُذْهِبُهُ بِالتَّوَكُّل
Verily, At-Tiyarah (omen) is Shirk; everyone might feel a glimpse of it, but Allah dissipates it with Tawakkul."
Allah said next