14 Now after John was arrested, Jesus came into Galilee, proclaiming the gospel of God, 15 and saying, “The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel.” Mark 1:14-15
The ministry of John the Baptist was intended to prepare people for the coming of Jesus. Once John was arrested for preaching that the king needed to repent, Jesus began his public ministry. His message built on that of John who proclaimed that the Kingdom of God was coming. Jesus announced that it was now here.
The reason the Kingdom was now here was because the king, Jesus, was now here. The people were looking for a messiah/king who would re-establish the glory days of Israel. But Jesus came as a king who would make known the glory of God. For that to happen people needed to turn away from their expectations of a military messiah and look for one who would instead conquer sin and death.
They also needed to turn away from every other area of sin in their lives and trust in the Good News message, the Gospel of the Kingdom. That Gospel was and is that we are broken and sinful human beings who have all run after our own personal idols. We are seeking salvation in things that only condemn us. But Jesus came to bring salvation to anyone who would turn from those idols and trust him. That is the heart of the Gospel. The God we reject each day by running after other gods, came into the world to conquer sin and death so we might live forever in his glorious kingdom.