The central mediaeval Christian defense of Scripture rested on the impossibility of mass falsification.

Evidence

Muslim case that he proposed to refute: ‘… one of your people would reply, I do not deny that the Jewish and Christian Books are of God, but (only) in the form in which they were written by their first authors’. The importance of this Islamic assertion was obvious to all who understood what it was. If the existing text of Scripture could not be vindicated, Islam would have a strong call upon the loyalty of those who loved Jesus, since it asserted a truer and purer devotion to Him. Peter of Poitiers would have had his Abbot start with this question, from which the whole great refutation of Islam which he planned would have proceeded. Neither Jews nor Christians could have falsified their Scriptures, he noted down: in many languages over all the world one and the same Gospel had been preserved. It was impossible that so many men of different languages and nations, especially the learned, could have concealed a falsification, or allowed themselves to be deceived, or mistaken false things for true. — Norman Daniel, Islam and the West, 276