In book of Mark, the first thing Jesus calls on us to do is repent. “...repent and believe in the gospel.” (Mark 1.14) If one does not repent then they really don’t believe. They are two sides of the same coin.
Read it again: repent and BELIEVE in the gospel. What is the gospel? That you are forgiven by receiving Jesus Christ apart from your works. New Testament repentance involves changing our mind about JESUS, not sin. Jesus has already taken care of the sin problem.