Mark of Jerusalem was one of Christianity's earliest leaders, a writer who recounted his own memories of the earliest days of the Christian church, and someone who committed the stories of Paul and Barnabas' missionary journeys to parchment. He was therefore, according to the First Doctor a "journalist".
Along with Matthew and Luke, he notably wrote an account of the life of Jesus Christ. However, his version differed from those of the other two because of a phenomenon the First Doctor called "chinese whispers". The First Doctor once got to see this "gospel of Mark" in a rough form, and found the experience incomparably exciting. (PROSE: Byzantium!)
Mark's journey
Having been only a boy at the time Jesus used Mark's mother's house for early meetings with his associates, Mark was not old enough to have been one of those original disciples. Consequently, his gospel, like Luke's, notably post-dated that of the disciple, Matthew.
Nevertheless, Mark did know Jesus directly. According to Amos, he tried to save Jesus' life when Judas betrayed them. After Jesus was subsequently arrested at the gardens of Gethsemene, he went into deep hiding. Eventually, Paul and Barnabas convinced Mark to join them on their missionary journeys, but they didn't immediately hit it off. After a bit of a break from Paul in Cyprus, the trio eventually reunited and founded curches in Galatia, Corinth, Athens and Ephesus. Though successful — largely because, according to Rayhab, they were allowed by "the will of the Christ" to perform miracles — these missions were not without opposition. The trio encountered stiff resistance from Hellenistic Jews and Zealots.
Still, once they reached Ephesus, they finally settled down for a bit. Here, Mark began his writing career and completed the gospel and other things that would become part of the Christian Bible. (PROSE: Byzantium!)
Behind the scenes
Byzantium! doesn't give its readers much in the way of identification for this individual. He's never described as "Mark the Evangelist", a name by which he is commonly known to real world Christians. He is, however, specifically called "Mark of Jerusalem" in the text of the novel.