Answer: Our first parents being left to the freedom of their own will, through the temptation of Satan, transgressed the commandment of God in eating the forbidden fruit; and thereby fell from the estate of innocency wherein they were created.₁

1. Genesis 3:6-8; Ecclesiastes 7:29; 2 Corinthians 11:3

For  further study see:

1. WSC 13: Did our first parents continue in the estate wherein they were created?
2. Elder Gregory Moeck of First Orthodox Presbyterian Church on WLC 21: “How could a man blessed with original righteousness sin?”