Mark uses three main titles for Jesus: Son of God, Messiah, and Son of Man.
Mark's Titles for Jesus
We're told that Jesus is the Son of God on several occasions: at his baptism
(1.9-11); during a couple of exorcisms (5.7); at the transfiguration (9.9); in
the words of the high priest (14.61); and in the words of the centurion at the
foot of the cross (15.39). If we take out the acclamations of the demons, we
have these words at the beginning, middle, and end of the gospel.
In the OT, Israel as a whole is sometimes referred to as God's son, as in Ex.
4.22. It's more common, however, for one of Israel's kings to be referred to
as God's son. See 2 Sam. 7.14 (also, Ps. 2.7, 89.26-7). It tends to refer to
political power.
Son of God
Messiah
Mark tells us that Jesus is the Messiah, the "anointed one." God's anointed
was traditionally thought of as a human king who would lead Israel to victory
over its oppressors. In Mk, traditional notions of messiahship are
reinterpreted. Jesus is portrayed as a messiah who suffers. We know of no
pre-Christian group that held a concept of a suffering Messiah.
Son of Man
In pre-Christian Judaism, Son of Man often meant "human being." In the OT,
"Son of Man" only appears in two places: throughout Ezekiel and in Daniel
7.The latter of these shows us an example of apocalyptic usage. Son of Man is
Jesus' self-designation in Mk. His reason for choosing this self-designation
is unclear. However, it certainly has apocalyptic overtones, as we can see by
reading Mk 13, which is called the "apocalyptic discourse." In this chapter,
Jesus tells of coming eschatological events, and of his role as the one who
will come "'in the clouds with great power and glory. Then he will send out
the angels, and gather his elect from the four winds, from the ends of the
earth, to the ends of heaven" (13.26-7). Mark uses the title "Son of Man" in
three ways: 1) to indicate Jesus' earthly authority; 2) to indicate Jesus'
future suffering; 3) to indicate Jesus future role as the eschatological judge
and redeemer.
There is no pre-Christian use of "Son of Man" to indicate one who suffers.
Mark seems to have interwoven prior concepts of the Son of Man with the
"Servant Songs" of Isaiah (42.1-4; 49.1-6; 50.4-11; 52.13-53.12). Isaiah 53
was especially significant for early Christian writers.
Mark has infused these terms with the notion of suffering. How does he do
this? By telling a story. He tells the story of Jesus, whom he identifies as
the Son of God, the Messiah, and the Son of Man, and in this story, Jesus is
alienated from all of those who are closest to him; he is consistently
misunderstood; he suffers and is crucified.