“Now the law came in to increase the trespass, but where sin increased, grace abounded all the more, so that, as sin reigned in death, grace also might reign through righteousness leading to eternal life through Jesus Christ our Lord.” -Romans 5:20–21
Paul now explains why the law was brought into the picture. In verses 13-14, Paul explains that men died before the law because sinned reigned even then. Without much explanation, he states that “sin is not counted where there is no law.” Here is where he picks up idea and finishes that thought.
One of the reasons the law came into the picture was to increase the trespass. Of course, that is not the only use of the law, but it is the law’s purpose in the context of Paul’s present argument. The law was given to be a mirror so men could see the true nature of his sinful condition and condemned standing before God. Calvin has a helpful analogy for this. He writes,
“He indeed teaches us, that it was needful that men’s ruin should be more fully discovered to them, in order that a passage might be opened for the favour of God. They were indeed shipwrecked before the law was given; as however they seemed to themselves to swim, while in their destruction, they were thrust down into the deep, that their deliverance might appear more evident, when they thence emerge beyond all human expectation. Nor was it unreasonable, that the law should be partly introduced for this end—that it might again condemn men already condemned; for nothing is more reasonable than that men should, through all means be brought, nay, forced, by being proved guilty, to know their own evils.”
The Law forces men to see how guilty, how evil they really are.
But where the law has increased sin (revealed it more clearly), grace abounded all the more. No matter how deep, how ugly the sin is, it can not exceed the depth and beauty of grace that leads men to eternal life through Jesus Christ our Lord.
Yes, sin brings death. But grace also grace also brings something: not just life, but eternal life. And this grace triumphs over death because it brings life, not through the law, but through the righteousness that can only be received apart from the law (Romans 3:21), through Jesus Christ.