"What shall we say then? Shall we continue in sin, that grace may abound? God forbid. We who died to sin, how shall we any longer live therein?"
Have you ever met someone who claims to be a Christian, yet is unrepentant about the sins in their life? I'm sure we all have. It's human nature to say, "God's going to forgive me anyway, so I might as well do what I want." As Christians though, we are called to put aside that sinful human response and learn to obey God's commands.
When we become Christians, we are called born-again. "Jesus answered and said unto him, Verily, verily, I say unto thee, Except one be born anew, he cannot see the kingdom of God." (John 3:3) Our old, sinful selves have been put to death and we are a new creation in Christ. This means we can't return to our old dead lives, but instead we get to start fresh.
Now, of course there will still be sin in our lives from time to time. No one will be perfected here on the earth. That will only happen in heaven. What this does mean though is that we should strive toward perfection. Every day, more and more, we should be relying on God to help us overcome temptation and banish sin. We shouldn't take God's grace for granted because although there is nothing He can't forgive, we should desire to please Him.