قُلْ إِنَّمَا حَرَّمَ رَبِّيَ الْفَوَاحِشَ مَا ظَهَرَ مِنْهَا وَمَا بَطَنَ وَالْإِثْمَ وَالْبَغْيَ بِغَيْرِ الْحَقِّ وَأَنْ تُشْرِكُوا بِاللَّهِ مَا لَمْ يُنَزِّلْ بِهِ سُلْطَانًا وَأَنْ تَقُولُوا عَلَى اللَّهِ مَا لَا تَعْلَمُونَ
Say: (But) the things that my Lord has indeed forbidden are Al Fawahish (great evil sins, every kind of unlawful sexual intercourse, etc.) whether committed openly or secretly, sins (of all kinds), unrighteous oppression, joining partners (in worship) with Allah for which He has given no authority, and saying things about Allah of which you have no knowledge. (Al-A'raf:33)
So thus the four haram (aforementioned) things are on different levels (of severity). It was started out by mentioning the least of them which was Al Fawaahish (great evil sins, every kind of unlawful sexual intercourse, etc.), and then secondly there was mentioned that which is more haram than it (i.e. Al Fawaahish), which is sins (of all kinds) and unrighteous oppression, then thirdly it was mentioned that which was more haram (than the first two) which is to make shirk with Allah The Most Glorious. Then it was mentioned fourthly, that which is more severe than everything that preceded; which is to speak about Allah The Most Glorious without knowledge. This is general whether one is speaking about Allah The Most Glorious without knowledge with regards to His Names and Attributes, or with regards to His Actions; as well as speaking about His religion and legislation (without knowledge)(1). Allah says,
وَلَا تَقُولُوا لِمَا تَصِفُ أَلْسِنَتُكُمُ الْكَذِبَ هَٰذَا حَلَالٌ وَهَٰذَا حَرَامٌ لِتَفْتَرُوا عَلَى اللَّهِ الْكَذِبَ ۚ إِنَّ الَّذِينَ يَفْتَرُونَ عَلَى اللَّهِ الْكَذِبَ لَا يُفْلِحُونَ
مَتَاعٌ قَلِيلٌ وَلَهُمْ عَذَابٌ أَلِيمٌ
And say not concerning that which your tongues put forth falsely: “This is lawful and this is forbidden,” so as to invent lies against Allah. Verily, those who invent lies against Allah will never prosper. A passing brief enjoyment (will be theirs), but they will have a painful torment.” (An-Nahl: 116-117)
So Allah threatened them with a severe punishment, (meaning) the one who lies about Allah with regards to His regulations. (For example), with regards to that which is not haraam (because in reality it is halaal) their statement, “This thing is haraam (forbidden)”, likewise with regards to that which is not halaal (because it is in fact haraam) their statement, “This thing is lawful” (and so on). So Allah The Most Glorious has clearly explained that it is NOT permissible for a slave to say that this is halaal or that is haraam except that he has to have knowledge that Allah has (in fact) made that thing halaal or that other thing haraam (before he utters such statements).
Some of the Salaf used to say,
So let one of you fear Allah when saying, “Allah has made permissible this thing or He has made prohibited that thing;” lest Allah says to you, “You have lied. I haven’t made this thing permissible nor have I made that thing prohibited.”
Thus, it is not proper to say about that which you have no knowledge of (this is halaal or that is haraam), when the revelation has come down clearly explaining and illustrating the halaal and the haraam. (So beware of saying) Allah has made this lawful and that prohibited while only relying on blind following and false interpretations.
TN: Sheikh Rabee and many others from the Ulema, in commenting on this verse, explain the reason forspeaking about Allah without knowledge being greater than shirk is due to the fact that shirk results and emanates from speaking about Allah without knowledge.