While it is obvious that God’s intention for law was both to curb sinful behavior and provide direction, its main purpose was to expose sin. Paul puts it this way in Romans 7:7: “I would not have come to know sin except through [law].” Law identifies and exposes sin because it sets the standard of God’s righteousness (His holiness) against that of our own self-centered, sinful motivations and behavior. So law can be thought of as a gauge of sorts, because it contrasts God’s righteousness with our own lack thereof.
But this begs a very pertinent question: did sin exist before the first law was ever given? Yes it did; at least man’s predisposition to sin did. But in the absence of law, sin laid dormant in Adam. Adam was created perfect, but being created perfect also meant being created human. And being human, he was subject to the chronic weakness of self-will. But Adam’s inherent self-will—and the sin which is the natural outcome of that weakness—was not recognized for what it was until law came on the scene to expose it.
Excerpted from: Free from the Power of Sin: The Keys to Growing in God in Spite of Yourself