14 Now after John was arrested and put in prison, Jesus came into
15 And saying, The [appointed period of] time is fulfilled (completed), and the kingdom of God is at hand; repent ( have a change of mind which issues in regret for past sins and in change of conduct for the better) and believe (trust in, rely on, and adhere to) the good news (the Gospel).
The Amplified Bible expands the meaning of believe. It entails more than knowledge. It is acting on that knowledge. What do you do with the understanding of the good news and how does it change your life? A little further in Mark, Jesus confronts a demon which recognizes Jesus as the Holy One of God. It knows and believes this to be true, but this is not the belief that saves. Repentance is lacking. The key is a change of conduct which is based on the trust in, reliance on, and adherence to the teaching and salvation offered by Jesus. This is active belief in the good news.