Do you remember when you first decided to become a Christian? Most of us do. In my case, it was as a young man. The thing is, I didn't really believe that I needed a savior. I didn't take my sin too seriously, and considered myself to generally be "a good person." The preacher had convinced me though that unless I acknowledged Jesus, I would burn in hell. So I chose Jesus.
The problem is that when we lose sight of the seriousness of sin - or never have it in the first place, we lose the thrill of forgiveness. Unless I understand how wrong I am, it is not possible for me to really appreciate being forgiven for that wrong. 2 Peter 1:9 tells us this, but I think most people miss the point. Really, how well do you know your own sin?