Marti tried to ground scriptural authority rationally but still used scriptural evidence.

Evidence

Marti’s emphasis on the truthfulness of Christ and of the Apostles, which the Qur’an maintains, as guarantee of the Gospel, was also original. For him, the zeal of Christians and the comparison of scattered manuscripts were equally proofs of Scripture; this was more traditional. Marti was above all anxious to establish the authority of Scripture on rational grounds, and yet he too defended Scripture with Scriptural data. — Norman Daniel, Islam and the West, 282