Mark's Gospel is believed to be the earliest written account of the life of Jesus. As Peter preached the gospel, Mark traveled with him and wrote down Peter's testimony. So what we have in the Gospel according to St. Mark are the eyewitness accounts of the Savior as experienced by Peter, Jesus' first disciple.
Mark is the shortest and most action-oriented gospel. He tells the story with relentless energy. It is written to a non-believing audience whose interest was not about theology, but power. It was as if they asked Peter, "I'm not interested in what Jesus taught. I want to know what he did." Mark's Gospel is the written account of Peter's response: "OK, I'll tell you exactly what he did, but sit down because this will 'blow you away.'"