A18: The sinfulness of that estate whereinto man fell, consists in the guilt of Adam’s first sin, the want of original righteousness, and the corruption of his whole nature, which is commonly called Original Sin; together with all actual transgressions which proceed from it₁.

1. Romans 5:10-20; Eph 2:1-3; James 1:14-15; Matt 15:19; Psa 51:5

For further study see:

1. WSC 14: What is sin?
2. WSC 15: What was the sin whereby our first parents fell from the estate wherein they were created?
3. WSC 20: Did God leave all mankind to perish in the estate of sin and misery?
4. WSC 84: What doth every sin deserve?
5. WSC 85: What doth God require of us, that we may escape his wrath and curse due to us for sin?
6. WLC 24: What is sin?
7. WLC 25: Wherein consists the sinfulness of that estate whereinto man fell?
8. WLC 26: How is original sin conveyed from our first parents unto their posterity?
9. WCF CHAPTER 6: Of the Fall of Man, of Sin, and of the Punishment Thereof
10. “The corruption of his whole nature” is also known as ‘total depravity’.  Here are some resources by Dr. R.C. Sproul:
    
11. Total Depravity (part 2) by Dr. R.C. Sproul:
    
12. Dr. Dennis Prutow explains WSC 17 & 18: