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.

Les explains from Romans 5:20 that the law entered in 1500 B.C., when Moses came down off the mountain and introduced the law to the nation of Israel. The verse says the law entered so that the offense might abound—not to make sin worse, but so that it might be seen for what it really is. Before 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.

Romans 5:20

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

What happened when the written Law came?

When the written Law came, it removed doubt: everything people did, even after searing their consciences, was breaking God’s Law and became sin. The Law entered so the offense might abound—not to make sin worse, but to reveal it for what it really is.