Medieval polemicists validated Scripture’s authority in order to quote it against Muslims.

Evidence

The reprobatio consists of two parts, the fourfold refutation proper (which I will call F’) and a shorter group of arguments (which I will call VY) calculated to validate Scripture on general and special grounds, and so enable the author to quote it henceforward even to Muslims. Most of V also forms the introduction to Marti’s explanatio, where in fact it fits better, since it should obviously always come at the beginning of a work. — Norman Daniel, Islam and the West, 95