The author used a variety of pre-existing sources, such as conflict stories (Mark 2:1–3:6), apocalyptic discourse (4:1–35), and collections of sayings (although not the Gospel of Thomas and probably not the Q source).
More fundamentally, some scholars believe Mark's reason for writing was to counter believers who saw Jesus in a Greek way, as wonder-worker (the Greek term is "divine man"); Mark saw the suffering of the messiah as essential, so that the "Son of God" title (the Hellenistic "divine man") had to be corrected and amplified with the "Son of Man" title, which conveyed Christ's suffering.
All this is in keeping with prophecy, which foretold the fate of the messiah as suffering servant.
Most scholars reject the tradition which ascribes it to John Mark, the companion of the apostle Peter, and regard it (and the other gospels) as anonymous, the work of an unknown author working with various sources including collections of miracle stories, controversy stories, parables, and a passion narrative.
As in all the gospels, the messianic identity of Jesus is supported by a number of themes, including: (1) the depiction of his disciples as obtuse, fearful and uncomprehending; (2) the refutation of the charge made by Jesus' enemies that he was a magician; (3) secrecy surrounding his true identity (this last is missing from John).
In Mark, the disciples, and especially the Twelve, move from lack of perception of Jesus to rejection of the "way of suffering" to flight and denial – even the women who received the first proclamation of his resurrection can be seen as failures for not reporting the good news.
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There is much discussion of this theme among scholars.