Medieval Christian polemicists uniquely claimed proof via another religion’s scriptures

Evidence

Fitzralph took the Scriptural defence further than any other author, either the Spaniards of a few generations before him, or the Cluniacs of a still earlier age to whom he owed his Islamic material. He thought that Christianity was alone among religions in being able to prove its case by the authority of another religion, and in spite of the fact that superficially the Christian religion was the most repugnant to reason. — Norman Daniel, Islam and the West, 297

Categorization

• XREF: connects to: Polemics could condemn a rival religion using only its own scriptures