What happened when the written Law was given?

When the written Law was given through Moses to Israel, it caused sin to abound in human experience, not by making sin worse but by revealing it as it truly is. Until then people only had conscience, but the Law removed all doubt that their actions were breaking God's Law and became sin.

Les teaches that the law entered in 1500 B.C. when Moses came down the mountain and introduced it to the nation of Israel. Until the written Law appeared, man could only go by conscience. But when the written Law came, there was no doubt that everything man was doing, even after searing his conscience, was breaking God's Law, and it became sin. The law entered so that the offense might abound in human experience—not to make sin worse, but so it might be seen for what it really is.

Romans 5:20

Moreover the law entered, that the offence might abound. But where sin abounded, grace did much more abound:

When and how did the law enter?

The law entered in 1500 B.C. when Moses came down from the mountain and introduced it to the nation of Israel. Until the written law appeared, people could only go by conscience; once it came, sin was clearly seen as breaking God's law.