I have issues discussing the Gospels as being 'credible' in any sense of the word, as it seems apparent, to me at least, that they are based on stories that preceded them.
For example, take 'Matthew's' depiction of Christ's birth, where Herod secretly plots to murder Jesus because he percieves the so-called Messiah as a direct threat. This same format of 'ruler intimidated by main character and seeks to eliminate him' is evident in a plethora of ancient stories of the time, associated to individuals such as Abraham, Sargon the Great, Augustus and Moses. This is simply a way of depicting a 'great birth to a great figure'. Who is to say Matthew's story is not just an invention based from preceding tales? The evidence is there.