On Quoting Scripture

A theologian's objection to my quoting verses from the Quran in response to questions about Islam, and my response to his reasoning: 

Theologian: I am not arguing on your respect of the other, but on the fact that you are using the Quran as an authority in the field of theology. The Quran itself contains historical ad hominem remark, which in my opinion might be blasphemous. Some passage seems well inspired, but some verses do not seem genuine to me. 

I did not make fun of your faith, but I am uneased by the use of any authoritative argument, in any domain. I prefer to reason without invoking revelations, because they mix the temporal and the atemporal. It might be done for private purpose, but it is not valid in public reasoning.

My Response: What then is the purpose of scripture if we do not study it and what is the use of revelation if we do not take guidance from it? And how do we know whether a scripture is or not a revelation if we do not check it with scientific research? Is not reason (theological philosophy) without scripture just wishful thinking? And what is the purpose of reason if it does not help us discover our purpose? And what is the use of knowledge if it doesn't lead us anywhere? 

I use Quran as an authority in the field of theology because I'm convinced that it is divine guidance for us. When I share some verses with non-muslims, the purpose is not to use it as an authoritative argument, which it is obviously not from a non-muslim perspective, but rather to let you evaluate it yourself. 

Scientific knowledge has and continues to be applied to improve the quality of our material life. What is so wrong with using it to help improve our understanding of the deeper questions of faith and purpose, and to improve the quality of our spiritual life? The scripture invites us to contemplate and evaluate all the signs around us to realize the veracity of the scripture, and consequently develop faith in the unseen, unverifiable knowledge that the scripture imparts to us. Why then should we insist on shying away from this invitation?