The word “gospel” was not a particularly Christian word at the time of the writing of the New Testament in the first Century Roman Empire.
Gospel was simply the word used for momentous news of a historic event that would no doubt bring a new situation or circumstances. For example, the news of a new Emperor being crowned was called gospel, as was the news of victory in battle by the Roman legions, ushering in a era of relative peace.
The gospel of the crowning of a new Emperor was good news if you were a senator in good standing with the new Caesar. The gospel was bad news if there was enmity and distrust between you and the Emperor-elect. When Mark decided to quill his life-story of Jesus, he introduced his papyrus with the words: The beginning of the gospel about Jesus Christ, the Son of God (Mark 1:1). Or to put it another way: Here begins my account of the momentous, historical news about the real king called Jesus.
This gospel is good news if you are willing to submit to Jesus and be part of his new kingdom. However, this gospel is extremely bad news if you refuse to submit to his rule by stubbornly continuing to life live your own way.
The words describing Jesus as “Son of God” where also particularly meaningful to Mark’s original readers. The common and official title of Augustus Caesar (Emperor at time of Jesus’ birth; nephew of Julius Caesar) was “Emperor Caesar Augustus, son of god.” Another inscription refers to Augustus as “The Emperor Caesar, son of god, Augustus, ruler of all land and sea.” A later Roman coin, said this of Emperor Tiberius, Augustus’s stepson: “Tiberius Caesar, son of the divine Augustus.” Emperors in the Roman Empire were worshipped as Gods and sons of God. We now call this religious phenomenon the “Emperor Cult”. Mark wrote into this cultural context and described Jesus as: the Christ, the Son of God.
Mark, who knew the apostle Peter well, wanted to “publish” the gospel concerning Jesus for a wider audience and for future generations. Mark, like Peter, was convinced that Jesus of Nazareth, not Caesar, was the ruler of all land and sea and the true Son of God. The rest of Mark’s book goes on to show us why Mark was convinced of these things.