Mark, also known as John Mark, was, of course, a Christian and Christians assume he was the author of the second Gospel, the Gospel of Mark. Written in Greek for a Christian audience, and using terms that were meaningless to non-believers (so true believers would only interpret what they wanted to interpret), Mark’s Gospel is a mixture of theology, history, and propaganda. It is written in a simple, forceful style, which hid the fact that it encouraged stupidity and self-deceit. Interestingly enough, self-deceit is referred to as 'faith' and actually has a positive connotation among most Christians. They thoroughly enjoy convincing themselves that their god exists when there is little to no evidence of his existence. They feel quite comforted that they'll go to a better place when they die in the holy wars that their religion created.
The Acts of the Apostles suggests that Mark himself accompanied Paul, that is Saint Paul, and Barnabas, his cousin, on their missionary journey (convincing people to stop thinking for themselves) Mark’s unexplained departure from that trip at Perga caused a later split between Paul and Barnabas, when Paul refused to take Mark on the second mission. Barnabas, in loyalty to his cousin Mark, broke off his relationship with Paul and traveled with Mark instead to Cyprus (Acts 15:36-39). Later Paul and Mark were reunited and went on an additional trip together. Mark was close to the older Peter, Mark likely acted as an interpreter for Peter and the Gospel of Mark is often considered to be Peter’s Gospel as transcribed by Mark.
As with the rest of the Bible we can't tell how much of Mark's Gospel or the Acts of the Aposteles is true. We can't tell if any of it is true.