sin?

another quote from C.S. Lewis' "Problem of Pain" (pp. 50-1):

A recovery of the old sense of sin is essential to Christianity. Christ takes it for granted that men are bad. Until we really feel this assumption of His to be true, though we are part of the world He came to save, we are not part of the audience to whom His words are addressed. We lack the first condition for understanding what He is talking about. And when men attempt to be Christians without this preliminary consciousness of sin, the result is almost bound to be a certain resentment against God as to one always inexplicably angry. Most of us have at times felt a secret sympathy with the dying farmer who replied to the Vicar's dissertation on repentance by asking 'What harm have I ever done Him?' There is the real rub. The worst we have done to God is to leave Him alone--why can't He return the compliment? Why not live and let live? What call has He, of all beings, to be 'angry'? It's easy for Him to be good!

To me it's sad when I see Christians who have a flippant attitude toward sin. How can you claim to be a Christian but not understand that sin is BAD?! That's paradoxical. Christians are supposed to be people who have realized FIRST that they are sinners and SECOND that God cannot tolerate sin. Then they learn that God made a way for our sins to be covered (by the shed blood of Jesus Christ), and in order for that blood to cover their sins they simply have to turn from their sins and have faith that Jesus' sacrifice was sufficient to pay for their sins. After this, they are covered with the blood of Christ and washed clean of their filthy sins and may enter into God's precense and have fellowship with Him and receive His gift of eternal life.

How can you travel down that path and become a 'Christian' if you have not FIRST realized that sin is bad? And then how can you, being a Christian, not continue to live as if it is bad?