In your comments of some of the given answers here you always use
as an argument, which seems to support your claim. After checking the Verses where this statement is quoted i came to the following explanation: It only means "Then will you not reason?" or "Don't you understand?" which is only a statement after Allah gave an explanation why something was a sin, not an order to think in general as you seem to have understood it: It has more to be understood as an order to make the right decision or implication of the given facts! A better argument -based on the Quran- in my opinion would be using expressions where following others blindly is declared as wrong ... maybe I may find better examples later! (This article in Arabic shows a couple of useful Verses)
Should we follow blindly or use our mind?
Battle of Karabala quoted a helpful Verse from the Quran. Which I would interpret as follows Allah and his Prophet (Peace be upon him) have shown us the right path and defined it for us. We can take it as is or try to understand it, for example by checking whether they go ahead with the goals of shari'a مقاصد الشريعة and
there are five fundamental goals these are the preservation of:
- Religion / Faith (din)
- Life (nafs)
- Lineage / Progeny (nasl)
- Intellect (‘aql)
- Property / Wealth (mal)
Now after this whether you commit a sin or not is your own decision! As if you checked it and still had doubts then asked a scholar as indicated by the Verse (21:7) and your doubts didn't vanish you still can try it!
Doubting is an adapted philosophy of Imam al-Ghazali some scholars accuse Descartes of plagiarism because his statement Cogito ergo sum or methodological skepticism seems to have its origin in al-Ghazali's statement which can be translated as "I doubt then I exist". And it's even said that Descartes had some books of him in his private library!
On the relation between Iman and committing a sin
In addition I would add the following statement of a Hadith which you may find in Sahih al-Bukhari and all the other six major Sunni Hadith Collections (Sahih Muslim, Sunan abi Dawod, Sunan ibn Majah, Sunan a-Nasa-i and Jami' at-Tirmidhi)
"The one who commits an illegal sexual intercourse is not a believer
at the time of committing illegal sexual intercourse and a thief is
not a believer at the time of committing theft and a drinker of
alcoholic drink is not a believer at the time of drinking. Yet, (the
gate of) repentance is open thereafter."
This statement seems to indicate that committing a sin is in first place a sign of weakness in the faith of a believer. This weakness can be temporary. Scholars interpret it as follows
that a believer doesn't commit a sin if his faith/belief is complete
... this is supported by an other Hadith (in Sahih al-Bukhari
-here an other Version- and Sahih Muslim and other Collections) on the authority of abu Dharr al-Ghafari (May Allah be pleased with
him). ... even if scholars are in consensus that this sins are major
sins, but nobody would be called a disbeliever if committing them ...
but believers with a lack of Iman (faith) ... if they repent their
punishment will disperse and if they died without repentance, Allah
may forgive them and let them enter paradise or he may punish them and
later let them enter paradise!
(This is taken from Imam an-Nawawi's al-Majmo' according this Arabic Fatwa)
So you can use your mind to try to understand why Allah has declared something as sin, I would even say you should, as some scholars used to say Allah loves/likes being worshiped by knowledge (not blindly followed). If you have strong Iman you may find out that these sins are bad and things which you shouldn't do!
There's no enforcement -at least a bit of- doubt is allowed
And finally if you read (2:256-260) you may find statements like
There shall be no compulsion in [acceptance of] the religion. The
right course has become clear from the wrong. ...
and how Allah dealt with doubtful people.
And Allah knows best!