The Messianic Secret refers to a motif primarily in the Gospel of Mark in which Jesus is portrayed as commanding his followers to silence about his Messianic mission. The Messianic Secret belongs in Mark's Gospel, but elements of it have been copied into the later synoptic gospels (Matthew and Luke) but not into John, which takes a very contrary view of Jesus. If it were found significantly in material believed to be sourced independent from Mark (particularly 'Q'), then the words of the Messianic Secret were probably those of Jesus, but this is not so.
Most explanations fall into two main categories - that Mark's reports of Jesus' commands to silence are historically true, or that they were a literary creation by Mark's author, perhaps being intended to draw attention to Jesus' Messianic mission by the repeated commands to silence on the issue.
Brennan Reed Hamil discusses the Messianic Secret at some length in a student paper written at the Lubbock Christian University. Firstly, he says, there have been historical explanations. In line with this approach, the motive of concealment has been seen as a facet of historical Jesus’ own behaviour and teaching, a characteristic of his, in other words, which is correctly reported by Mark.
Hamil says there remain many legitimate questions to consider, none of which seem to have definitive answers. This device of the author of Mark perhaps shall never be fully understood. Regardless of — perhaps because of — the curious and mysterious nature of the secrecy motif, it persists in the minds of Markan scholars.
Oxbridge Notes (Is There A Messianic Secret In Mark Notes) looks at whether the Messianic Secret can be explained historically and puts forward the contention that Jesus wished to conceal his messiahship from men and women during his ministry for fear that it would have been misunderstood as a claim to political kingship, thus choosing the title ‘Son of Man’. The explanation is then criticised, in part because it leaves unresolved the problems of how the bystanders could ignore the confessions of Jesus’s identity made by the men and women who were possessed by these spirits.
MacDonald's hypothesis would be easy to dismiss except that he has built a sound reputation among biblical scholars, and the reviews that I have read for The Homeric Epics and the Gospel of Mark describe his hypothesis as sound, although sometimes overstated.
The Messianic Secret belongs in Mark's Gospel, but elements of it have been copied into the later synoptic gospels (Matthew and Luke) but not into John, which takes a very contrary view of Jesus. If it were found significantly in material believed to be sourced independent from Mark (particularly 'Q'), then the words of the Messianic Secret were probably those of Jesus, but this is not so. For whatever reason, the association with Mark makes the author of this gospel the most likely source of the Messianic Secret, rather than Jesus.