The Gospel According to Mark

Mark is the shortest of the four Gospels, due to his audience being mostly Roman. When one is preaching to an imperial police state, one is not concerned about making connections to a Hebrew past, nor does one make lengthy philosophical points. Romans wanted action. This is why the Gospel According to Mark has fewer sermons and more movement. It's a continuous narrative about Jesus's ministry, which would have appealed better to the Romans than any of the other Gospels.

Mark explicitly worked Romans into his Gospel, as well. At Jesus's crucifixion, Mark wrote of a Roman centurion (a commander of 100 soldiers) who proclaimed, "Truly, this was the Son of God" (Mk 15:39). Mark's Roman audience would have definitely perked up when they heard this part of the story, since it was an act of faith by one of their own.

Much like Luke, Mark wasn't one of the 12 apostles. Instead, the two were among the first 72 disciples. The apostles were there to witness Jesus's words and deeds, but disciples learned about it secondhand, told by other sources. Luke is believed to have received much of his information from Mary, the mother of Jesus, and Mark from his friend Peter, the chief apostle.