I'm in the process of completing and reviewing the late Martin Hengel's tremendous 1977 work, Crucifixion. Hengel was a German New Testament scholar who, at the time of his passing, was the Emeritus Professor of New Testament and Early Judaism at the University of Tubingen.
Here is a very interesting 4-part video interview with Dr. Hengel in which he speaks to the question of the authority of the Gospels in early Christianity.