Fourthly, I do not find that the Quran teaches that the Quran cannot be read by a professed non-Muslim. I use the word “professed non-Muslim” for two reasons. Firstly, many people profess to be, for example, a Muslim, a Buddhist, a Christian, etc but they never practice nor, sometimes even understand Islam, Buddhism or Christianity. Secondly, in the same vein, I have met people who profess to be a non-Muslim but I see that substantially and in substance, they are good Muslims because they are practicing the fundamental teachings of the Quran! Thirdly, I have also met people who profess to be Muslims but have no desire/interest to know what the Quran teaches them.
Surely, on Judgment Day, the Creator is not going to ask for your passport/IC to determine whether you have lived you life righteously. Superficialities and formalities will count for nothing. It is substance that will matter. (On earth, we busy ourselves with superficialities and futile, childish formalities in the name of procedure, management and protocol. Another form of divisive and inhibitive culture.)
On the other hand, I find that the Quran addresses almost all categories of people - the believers, the disbelievers, mankind in general, the rejecters, the hypocrites, the seeker, the doubter, the ignorant, the knowledgeable, the rich and the poor, everyone. The Ad-Deen approach will enable the acceptance of a non-Muslim reading the Quran or asking questions or even debating about it. If you do not inquire how do you know and learn?
“We have sent you as a bearer of glad news and a warner to the whole of mankind, but most people do not know.”. (Quran: 34:28)
“O people! surely the Messenger has come to you with the truth from your Lord, therefore believe, (it shall be) good for you and If you disbelieve, then surely whatever is in the heavens and the earth is Allah's; and Allah is Knowing, Wise”.
“We have explained in detail in this Qur'an, for the benefit of mankind, every kind of similitude: but man is, in most things, contentious”. (Quran 18:54)
The religious approach on the other hand does not allow this. Only the professed Muslim can read the Quran. It puts some of the professed Muslims on the defensive and a proprietary mindset ie “the Quran is my property and you the professed non-Muslim have no business reading or talking about it”. The religious folks particularly want to have a monopoly over their religion especially if it gives them power, recognition, position or wealth. This is one of the reasons why you find that politicians tend to be religious, complete with the entire paraphernalia of religious qualifications, religious appearances, symbols and dressing.
These superficialities sell very well because the religious approach already would have brainwashed you not to question these folks. For those who read the Quran, this is not surprising at all as the Creator has already warned us about such tricks.
“Do you not see those who claim sanctity (purity) for themselves? Nay- Allah sanctifies (purifies) whom He pleases. But never will they fail to receive justice in the least little thing”. (Quran: 4:49)
Surely if the Quran is from the Creator it must be for all the created who want to read research or debate it? The Creator says that the Quran is a mercy for all mankind. Surely all mankind is entitled to this mercy from the Creator?