Who Were Mark and Luke? From Luke 6:13–16 (NIV):


  When morning came, he called his disciples to him and chose twelve of
  them, whom he also designated apostles: Simon (whom he named Peter),
  his brother Andrew, James, John, Philip, Bartholomew, Matthew,
  Thomas, James son of Alphaeus, Simon who was called the Zealot, Judas
  son of James, and Judas Iscariot, who became a traitor.


John and Matthew were included, but the remaining two authors of the Gospel – Mark and Luke – were not.  Why is this, and how did they come to know Jesus so closely as to later write his Gospel?
Historically, Mark is seen as one of Peter's disciples.  The historian Papias in the 2nd Century refers to him as such.  Likewise, the evidence in the narrative, for example, indicates that Peter was a significant source for most of the material, and most theologians accept Mark as "Peter's" Gospel.  Also, its seeming indication that the Temple is still standing marks it as an early Gospel.

Luke was Paul's travelling companion.  From very early on, this is what the church has taught. Luke and Acts were clearly written by the same individual (See Acts 1), and Luke is the traditional author.