"and saying, the time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel"
This is Jesus' message. The kingdom of God is ready to invade this realm. Your appropriate response is to repent, change your wicked ways, and embrace the truth of the good news and act as though it were true. Your actions give evidence of what you truly believe.
It was an act of faith that Jesus was asking the people who heard Him to commit to. The kingdom had not yet appeared openly or manifestly so that it was undeniable. He was the representative of the Kingdom, but it was still an act of faith. They had to embrace a truth that was not yet evident.
The nature and function of belief is a fascinating study. It always precedes the manifestation of what is believed. Belief is reaching into the future and grasping something mentally and spiritually that has not happened yet and holding it as though it were already true.
Belief means holding to a different future than you are presently experiencing or headed toward and submitting that different future to the Lord for actuation and allowing Him to move you toward that new future that you didn't deserve and that would not have happened.