The combination of John’s preaching, Jesus’ gospel, and the conclusion of Mark 1:15, suggests that, thanks to Jesus Christ, the kingdom is now accessible. You can go in. John’s call to repent remains in effect for sinners: Jesus repeats it (Mark 1:15: repent and do penance). But he adds something to it: believe in the gospel (now preached by me).
After John, the gospel is now current: the kingdom of God is not just something that is coming, but it has also become accessible. Whoever looks around, only sees Jesus. From the beginning, this preaching has been mysterious: it must point in the direction of faith in this Jesus, who comes after John and unlocks the kingdom that is near.1
15 and saying, “The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel.”