proto-Mark <---- proto-Luke (aka Mcn) <------ Luke <----- Matthew
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According to Paul-Louis-Couchoud:
proto-Luke (aka Mcn) <----> Mark <----- Luke <---- Matthew
Often it is said that proto-Luke cannot be the Earliest Gospel since:
it is not so allegorical and secret as Mark (therefore assuming already a historical faith and a popular certainty in what it talks about)
About that secrecy in Mark, I see only three possible reasons behind it:
1) the Secret is Messianic (Wrede), to mean that the people don't know that Jesus is the Jewish Messiah, when really Jesus is Messiah.
2) the Secret is not Messianic (Ory), to mean that the people believe that Jesus is the Jewish Messiah, when really Jesus is not the Jewish Messiah.
3) the Secret is anti-marcionite Apology:
The god of the Jews said, Aure audietis et non audietis (Is. vi. 9). Jesus, on the other hand, wishes all ears to be opened (T. iv. 19). All should listen, since there is no longer anything hidden; everything is made clear.
(Couchoud, Creation of Christ, p. 399)
In 2 on 3 cases, the Earliest Gospel is marcionite in nature.