Christian polemicists argued the Koran proves scripture’s divinity through its own claims

Evidence

In order to prove the validity of the Jewish and Christian scriptures, Peter starts from the normal Christian viewpoint that Koranic stories of, say, Abraham or Noah are corrupted versions of their biblical counterparts; the marginal annotations in Robert of Ketton’s translation of the Koran re- flected this notion. Peter says he was amazed to find that Muhammad, in the Koran, had mixed elements from Christian and Jewish scriptures and moreover had praised those scriptures. Assuming, rather than arguing for, the primacy of Judeo-Christian scripture, he affirms that if these scriptures are divine, they should be accepted wholly, not in part; if they are not divine, they should be rejected wholly, not in part (Contra sectam, §57). — John V. Tolan, Saracens, 862