Mark just spent two whole verses on Jesus's temptation in the wilderness. Now, Jesus rises in full force. Now that John the Baptist has proclaimed to the world that Jesus is the Christ, God allows for him to be arrested and Jesus says, "The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel."
For centuries, Israel had looked to many men to save them from their enemies: Samson, Gideon, Samuel, David, Solomon, Josiah, Isaiah, Elijah, Hezekiah, Daniel, and even John the Baptist, the last Old Testament prophet. None of them led sinless lives and none of them could save lost souls from their sins.
Finally, the last prophet before Jesus comes has fulfilled his purpose and will soon lose his head, and Jesus comes on the scene as God's answer to the needs of Israel and the whole human race.
He then proceeds to call disciples to himself. Many people did this in Jesus's day. They would call disciples and be their rabbi, and those men would follow that man for a certain amount of years. Before Jesus came, rabbis would teach under the name of various famous men such as Hillel, Gamaliel, etc. They never spoke of their own authority. Jesus changed that and spoke from his own authority. And then God verified his authority by performing miracles. He healed a demon-possessed man and then proceeded to heal Simon Peter's mother-in-law of an illness.
Many people today look forward to heaven and define "gospel" in many ways, but heaven is not a place and gospel is not a music style. They are both one man, and that man is Jesus Christ, God the Son, sent to live the only perfect human life and to give the only perfect sacrifice for the sins of those who believe in him, himself. This is why Muslims and Jews do not worship the same God as me. They refuse to believe that Jesus is the one sent from God to save people from their sins, and they will not find salvation from their sins apart from repenting and believing in Jesus alone.