What does Mark 1:15 mean?

Mark's first direct quote of Jesus is the announcement that God's kingdom has arrived. The gospel message includes a sense of urgency, especially with Jesus live and in-person among those He taught. They are no longer to look forward to a future fulfillment of the Messianic prophecies; Jesus is now on the scene fulfilling God's words among God's people (Galatians 4:4). Scholars number hundreds of prophecies in the Old Testament about the coming of Israel's savior. Despite this, the Jews had a very inaccurate idea of what the Kingdom of God was and what the Messiah would do for them. They believed the Messiah would lead them into political independence, civil peace, and personal prosperity. Instead of a military champion, Jesus came as meek as a lamb (John 1:29), not to remove the barrier between the Jews and prosperity, but to remove the barrier between all of humanity and God Himself.

John the Baptist's message was to repent and believe that salvation was coming. Jesus' message is to repent and believe in the salvation that is now here. To repent, from the Greek metanoeo, is to change one's mind and agree that one's sins are abhorrent. It is not only remorse for sin but also a changed attitude that chooses to live for God. To believe, from the Greek pisteuo, means to be fully convinced. To "believe" and to "repent" go hand in hand. It is impossible to fully believe in Jesus' message of salvation and not turn away from our sins (1 John 3:4–6). Christians will never be perfect on this earth, but our inclination to live a sinful life changes into a desire to live for God.