So far, we've seen that Mark highlights Jesus trying to keep his activities and identity somewhat hidden. In fact, while Jesus clues his disciples in and they believe he is the Messiah, they do not understand that for Jesus to be Messiah means for him to die. This observation brings us to the third and most important distinctive of Mark's presentation. The cross was the focal part of Jesus' mission.
Mark teaches the resurrection...
However, at least as we have it, the resurrection is more like the epilogue of the Gospel of Mark. The climax of Mark, the point of the story where the tension reaches it highest point before being released, is at the crucifixion. Indeed, it is with the confession of the soldier by the cross.  Three times in the latter pages of Mark, Jesus predicts his impending death. Three times the disciples don't get what he is saying.
But the reader can sense the foreboding. The disciples may blissfully think they are going to Jerusalem to be part of the renewed kingdom, but the reader knows what is coming. The reader can hear the ominous music in the background. The tension is rising, building, and there is a sense of inevitability.
After Jesus and his disciples have started toward Jerusalem--and after Jesus has made his third prediction that he is going to die--James and John obliviously ask Jesus if they can have thrones on either side of him in the kingdom. The end of his response give us the only real substantial peek into the significance of Jesus' death for Mark: "the Son of Man did not come to be served, but to serve, and to give his life as a ransom for many." This is effectively the purpose statement of Jesus' earthly mission in Mark.
We naturally read a statement like this one now and think of the incarnation, the fact that Jesus existed before he was on earth and "came" down to earth.  Experts on Mark debate whether this is exactly what Mark was thinking, since Mark doesn't clearly speak of the incarnation elsewhere. But as Christians it is fine for us to read it this way, since we believe in the incarnation. In either case Mark is telling us that the focal point of Jesus mission was to die for the sins of many.
No doubt those who first heard this saying immediately thought of some in Israel as the "many." But Mark was not likely written for a non-Jewish, Gentile audience. When Mark 7 is presenting Jesus' encounter with some religious leaders over washing hands, Mark takes the time to explain what the Jews do to keep from becoming ceremonially unclean (7:3). Mark speaks of Jews in the third person, as someone else, not as if the audience itself is Jewish.
Similarly, it is interesting that Matthew, the most Jewish gospel, does not mention Mark's conclusion that Jesus was implicitly declaring all foods clean when he said that unclean was something that came from the inside out, not from the outside in (Matt 15:17; cf. Mark 7:19). We cannot know for certain, but is it possible that such a conclusion was far more controversial for Matthew's possibly Jewish audience as opposed to Mark's primarily Gentile one?
So when Mark tells us that Jesus came to die for many, it potentially included everyone, not just Jews. We all know what a ransom is from the movies, but we shouldn't push the image too far, as some in Christian history have. God is not paying off Satan to get humanity back--or some other overreading of the analogy. Jesus is simply dying to free us from the consequences of our sins and the sins of those before us.
We get the barest hint of what this means when the temple curtain rips in half in Mark 15:38. The temple provided atonement for sins. Perhaps this is Mark's version of Hebrews. Atonement for sins is now accomplished.
The rest of the New Testament also fills in some of the details for us as Christians. We cannot know entirely what Mark himself thought Jesus was freeing us from. The earliest Christians may have thought Jesus' death was freeing Israel from the consequences of its sins as a nation enslaved to foreign powers like the Romans. Acts 1:6 mentions the disciples asking Jesus if he was then going to restore the kingdom to Israel.
But in Paul's writings we get a more universal sense of Sin and its power. Jesus' death frees us from the power of Sin in this life (e.g., Rom. 8:1) and from the power of death in the next (cf. 1 Cor. 15:54-55; Heb. 2:14). Mark does not go into such details. His ministry brought freedom from the power of demons or from the power of sickness, but his death is surely about a much more profound liberation still!
The cross in Mark is therefore not the low point of the story. In a massive reversal of significance, the cross turns out to be the high point of the story, the climax. This is the ransom taking place. This is the moment of liberation from the consequences of sins. The centurion by the cross gets it, a non-Jew of all people.  He sees how Jesus died and immediately comes to the right conclusion: "Surely this man was the Son of God!" (15:39).
Far from showing Jesus was a fake, Jesus' death on the cross was the very focal point of his earthly mission in Mark. After Peter confessed Jesus as Messiah, he rejected Jesus' subsequent prediction that he was going to die (Mark 8:31-32). But Jesus in turn made it clear that it was Peter who didn't understand how things worked. The cross would in fact be the high point of Jesus' mission as Messiah. And though Peter and his disciples did not get it, the Roman centurion did.
 Cf. Rhoads and Michie, Mark as Story.
 Cf. Simon J. Gathercole, The Preexistent Son: Recovering the Christologies of Matthew, Mark, and Luke (Grand Rapids: Eerdmans, 2006).
 Again, cf. Mark as Story.