At this point, my friend, I must try to make it quite clear what sin is. Sin is not merely doing wrong things like stealing, or committing adultery, or getting drunk, or committing murder. Sin is basically separation from God, and is anything against the holy will of God. Not only is an evil deed sin; pride and envy and hatred and lustful thoughts also are sin. Jesus Christ said that the two greatest commandments of God are to love God with all one's heart, and to love one's neighbor as one's self. Since these are God's chief commands, then failure on our part to love God perfectly and to love others as we love ourselves, are the greatest sins. Is there anyone who has kept these commandments perfectly? No, all have broken them except the sinless person, Jesus Christ. He lived a perfect life, and that is what God requires of us.  His commandment is: "You, therefore, must be perfect, as your heavenly Father is perfect." (Matthew 5:48).

But how can we sinners, whose hearts are full of evil, become perfectly pure and good as God commands us to be? A man who has a cancer does not need good advice about keeping the laws of health. What he needs is a skilled doctor who is able to cure him. Just so, a man sick with the disease of sin needs more than good laws and good moral sayings telling him what to do and what not to do; rather, he needs a spiritual doctor who is able to created in him a new mind and heart and will, and to give him the power to do what god requires of him.

Read full article here: What Was Christ's Work On Earth?- An Explanation For Muslims