The Westminster Shorter Catechism

Q18. Wherein consists the sinfulness of that estate whereinto man fell?

A. 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 it.[1]

Proofs

[1] Romans 5:10–20; Ephesians 2:1–3; James 1:4, 15; Matthew 15:19; Psalm 51:5.

Comments

This question addresses the extent of the sinfulness which was brought upon every individual descendant of Adam as a result of his Fall. This sinfulness consists of two parts, which can be diagrammed for clarity thus:

I. Original Sin

(a) Imputed Guilt of Adam
(b) Inherited sin nature of Adam

i. want of ability and desire to do right
ii. positive inclination to sin

II. Actual Sins

Notice that Original Sin does not refer to the sin of Adam or Eve in their eating of the forbidden fruit. Rather, it refers to the result of their sin on their posterity, which comprises two parts, namely: (a) guilt imputed or accounted on them, since Adam was our representative; (b) sin nature inherited, since we descend naturally from Adam. This sin nature is described in the answer positively as a “want of original righteousness” and negatively as “the corruption of his whole nature.” It is because of this sin nature that actual sins spring from our heart.