Kyle Keefer, The New Testament As Literature: A Very Short Introduction (OUP, 2008), 108.
Reading these two gospels [i.e. the Gospel of John and the Gospel of Mark] together presents a series of dilemmas. Could both of these be true? Is one more historical and the other fanciful? Does John’s account try to correct what he sees as an embarrassingly weak Jesus in Mark? To read the gospels together means coming to grips with contradiction. One way to deal with it is to harmonize the two accounts. That is, assert that Jesus did truly suffer in John and that he covers it up or, conversely, that he does not truly agonize in Mark. He prays to God with only a minimal amount of pleading, and that is his slight nod to a natural human impulse. Both of these harmonizations clearly violate the narratives by not taking either seriously. Another option, one taken by historians, is to claim that Mark’s Jesus stands close to the historical circumstances, with an emphasis on Jesus’ humanity and that John’s Jesus has no historical basis. John has theologized Jesus, making him divine rather than human. This reading only works by giving priority to Mark and subjugating John. Both options distort the text by imposing nontextual criteria on the gospels. How then can both of these be read together without making one subservient to the other?
If we think of the gospels as portraits, the problem of inconsistency dissolves.