Original sin is the inherited corruption of our entire nature and the imputation of Adam's guilt, which renders all humanity spiritually dead and inclined toward evil from conception. It is not merely an imitation of Adam's sin, but an inward, hereditary disease that can only be remedied by the grace of God in Christ.
Original sin is not simply the actions we commit; it is the fundamental state of our existence since the Fall. As stated in the 1689 LBCF Ch.6 §3, Adam and Eve stood as the root of all mankind, and through their disobedience, they conveyed a corrupted nature to all their posterity. This includes the imputation of Adam's guilt, the loss of original righteousness, and a pervasive corruption of our entire nature—our understanding, heart, and will—rendering us indisposed to all that is spiritually good (Westminster Shorter Catechism Q.18).
While original sin marks us as "children of wrath" by nature, we find our hope entirely outside of ourselves in the finished work of Jesus Christ. The Scripture declares, "Therefore, as through one man sin entered into the world, and death through sin; and so death passed unto all men, for that all sinned" (Romans 5:12). We are indeed more sinful than we ever dared believe, but we are also more loved in Christ than we ever dared hope. It is only because the Lord Jesus sets us free that this hereditary debt is cancelled for the believer (1689 LBCF Ch.6 §3). Our obedience is not the cause of our acceptance, but the thankful response to the righteousness that has been freely credited to us through faith in Him.