If Allaah (The Creator and Originator of Everything) did not send down any authority for their creeds, beliefs and deeds, then what are its sources?!
In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Allaah (The Most High) informed us that Yusuf (alayhis-salaam) said to his companions in the prison:
يَـٰصَٮٰحِبَىِ ٱلسِّجۡنِ ءَأَرۡبَابٌ۬ مُّتَفَرِّقُونَ خَيۡرٌ أَمِ ٱللَّهُ ٱلۡوَٲحِدُ ٱلۡقَهَّارُ
مَا تَعۡبُدُونَ مِن دُونِهِۦۤ إِلَّآ أَسۡمَآءً۬ سَمَّيۡتُمُوهَآ أَنتُمۡ وَءَابَآؤُڪُم مَّآ أَنزَلَ ٱللَّهُ بِہَا مِن سُلۡطَـٰنٍۚ إِنِ ٱلۡحُكۡمُ إِلَّا لِلَّهِۚ أَمَرَ أَلَّا تَعۡبُدُوٓاْ إِلَّآ إِيَّاهُۚ ذَٲلِكَ ٱلدِّينُ ٱلۡقَيِّمُ وَلَـٰكِنَّ أَڪۡثَرَ ٱلنَّاسِ لَا يَعۡلَمُونَ
“O two companions of the prison! Are many different lords (gods) better or Allah, the One, the Irresistible? You do not worship besides Him but only names which you have named (forged), you and your fathers, for which Allah has sent down no authority. The command (or the judgement) is for none but Allah. He has commanded that you worship none but Him (i.e. His Monotheism), that is the (true) straight religion, but most men know not.” [12:39-40]
[يَـٰصَٮٰحِبَىِ ٱلسِّجۡنِ ءَأَرۡبَابٌ۬ مُّتَفَرِّقُونَ خَيۡرٌ أَمِ ٱللَّهُ ٱلۡوَٲحِدُ ٱلۡقَهَّار- O two companions of the prison! Are many different lords (gods) better or Allah, the One, the Irresistible?] Meaning: Lords that are deprived of the ability to act and are weak-neither able to benefit nor harm; neither able to give nor withhold. And they are various types, (such as) trees, stones, angels and dead people and other than them of those various objects of worship that are taken as objects of worship by the polytheists. Are those objects of worship better or Allaah? [Allaah]–The only One Who has Attributes of absolute perfection; ٱلۡوَٲحِدُ The one singled out in His Essence, His Attributes and Actions, and He has no partner in any of that.
[ٱلۡقَهَّارُ] The Irresistible- the one to whom everything has submitted due to his absolute subjugation of everything and His absolute dominion over everything–whatever He wills takes place and whatever He does not will cannot take place; (and) مَّا مِن دَآبَّةٍ إِلَّا هُوَ ءَاخِذُۢ بِنَاصِيَتِہَآۚ– There is not a moving (living) creature but He has grasp of its forelock.
It is known that the one whose affair and description is as such is superior to (those) different gods, which are mere names–neither do they possess perfection nor actions. So because of this [Allaah (The Most High) said that Yusuf (alayhis-salaam) said to them]:
[مَا تَعۡبُدُونَ مِن دُونِهِۦۤ إِلَّآ أَسۡمَآءً۬ سَمَّيۡتُمُوهَآ أَنتُمۡ وَءَابَآؤُڪُم— You do not worship besides Him but only names which you have named (forged), you and your fathers]; Meaning: You gave them names and called them gods, whilst they are nothing. They have no attribute of Uloohiyyah (i.e. attributes of absolute perfection-free from all deficiencies, weakness etc) and [مَّآ أَنزَلَ ٱللَّهُ بِہَا مِن سُلۡطَـٰنٍ] – for which Allah has sent down no authority; rather Allaah has sent down authority in prohibiting them (i.e. worship of these things) and clarified its falsity.
And if Allaah did not send down any authority for it, then neither can it be a (sound) path nor a means (of worship), nor does it have any proof. That is because to Allaah (alone) belongs the judgement and He is the one who gives commands and prohibitions. And He (alone) is the One Who legislate the divine laws and rulings, and He is the one that commanded you أَلَّا تَعۡبُدُوٓاْ إِلَّآ إِيَّاهُۚ ذَٲلِكَ ٱلدِّينُ ٱلۡقَيِّمُ– That you worship none but Him (i.e. His Monotheism), that is the (true) straight religion]; Meaning: The straight religion that leads to every good and the other religions besides it are not upright; rather they are crooked and will lead to every evil.
[وَلَـٰكِنَّ أَڪۡثَرَ ٱلنَّاسِ لَا يَعۡلَمُونَ –But most men know not] the reality of affairs, even though the distinction between worshipping Allaah alone and associating partners with him in worship is a clear and manifest affair (i.e. there is no doubt about the falsity of worshipping others and the absolute proof that Allaah alone has the right to be worship). However due to many people not possessing knowledge of this affair, they fall into shirk.
[Slightly paraphrased…For Further details see Tayseer Al-Kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan]