What makes the Qur'an authentic?

In a Nutshell: The Qur'an is self-authenticated as an everlasting miracle both in its linguistic rhetorical structure and the merits of its sublime teachings and meanings. God explicitly declared in several places in the Qur'an that whoever doubts the legitimacy and authenticity of the Qur'an should bring a surah like it (e.g., Qur'an 10: 38). If the pagans at the time of the prophet could bring a surah like those of the Qur'an, they would have done so since it was the easiest way to defeat Islam. History shows us they were unable to. They ultimately had little choice but to acknowledge the miraculous nature of the Qur'an and their inability to make a masterpiece such as any of its suras. From the advent of Islam, many examples testifying to this are documented in Islamic history. In the seerah one night Walid ibn Mugirah came secretly to the Prophet's (saw)house to listen to his recitation in order to know something about the nature of the Qur'an. The more he listened the further he was impressed by Qur'anic fluent verses. He was so captivated by the Qur'an, as he came for two more nights. The next day, when he was asked about the Qur'an, in response he said: والله لقد سمعت آنفا من محمد کلاما ماهو من کلام الانس و لا من کلام الجن. ان له لحلاوة و ان علیه لطلاوة و ان اعلاه لمثمر و ان اسفله لمغدق و انه یعلو و لایعلی " (ر.ک: تفسیر طبری،ج 29، ص 98. سیره ابن هشام،ج 1،ص 288. ابن ‏اثیر: اسد الغابة،ج 2، ص 90) I have just heard from Mohammad a word which is neither the word of man nor the word of Jinn. It is much well-structured and sweat. The branches of which are fruitful and its roots are much firm and consistent. Many were there throughout the history who in spite of their hostility to the Qur'an and the Prophet (saw) have acknowledged the linguistic excellence and merits of the Qur'anic verses. It should not be lost sight of that the miraculous nature of the Qur'an must be studied and put in to test by the experts and rhetoric scientists.