When the law comes home to the sinner's mind and when God's claims are forced upon the sinner's notice then, Paul says, the law is doing a wonderful work. It is revealing one's own sin to oneself. It is the law revealing sin in its sharpness that prepares for grace to abound. Grace would not abound, indeed grace would not even be grace, if it were not for the reality of sin. It would be an insult to offer mercy to innocence.
Innocence needs no mercy.
It is sin that requires mercy and in the case of sinful men before a holy God the mercy required is an absolutely gratuitous mercy. There is nothing in the individual to call forth the mercy of God. It is not even our need and our lostness that make God move to save us. It is not anything in our nature. It is the nature of God that givesgrace.