The second part of Mark 1:15b reads: "...and the kingdom of God is at hand; repent and believe in the Gospel." The kingdom of God is at hand. Jesus came to earth to inaugurate the kingdom of God. His incarnation began the establishment of God's rule on earth. Over 2,000 years the message of the Gospel, God's Good News of the forgiveness of our sins and his love ruling has spread over all the world. The kingdom's rule and reign will be completed when Jesus returns, totally eradicates all sin and evil, and reestablishes Eden, as God the Father originally intended.
Until then, people of Jesus' kingdom are called to repent and believe. First, we're called to believe the kingdom of God thru Jesus is true. We're called to accept his free gift of forgiveness for our sins by grace thru faith. We are to believe he is God, ruling this world from heaven and our hearts within. Once forgiven, then we're to repent. The best definition of "repent" I've ever heard is "Stop it!" We desire to stop our sin. We desire to be holy and live as he desires us to live as his kingdom citizens. We're a holy people, a set apart priesthood. We believe in him and we repent of our sin. When we do, it is evidence that the kingdom of God is at hand. It's now and here in our lives.
Repent and believe: two simple evidences we've accepted the kingdom of God.