Sin is defined in Scripture as any lack of conformity to, or active transgression of, the holy law of God. It is a fundamental turning away from our Creator that renders us, in our natural state, dead in trespasses and deserving of divine wrath, yet this reality makes the grace of the Gospel even more glorious.
At its heart, sin is defined by its relationship to the character and law of God. As the Westminster Shorter Catechism — Question 14 notes, it is any 'want of conformity unto, or transgression of, the law of God.' The Apostle John provides the biblical foundation for this, stating, 'Every one that doeth sin doeth also lawlessness; and sin is lawlessness' (1 John 3:4). To sin is to reject the governing authority of God and to deviate from the standard of holiness He has established for His creation.
The 1689 LBCF Ch.6 §1 clarifies that by this sin, our first parents fell from their original righteousness, and this corruption is now propagated to all their descendants. We are, as the Apostle Paul reminds us, universally affected: 'for all have sinned, and fall short of the glory of God' (Romans 3:23). While Reformed traditions acknowledge that all sins bring us under the curse, they also distinguish—as seen in Westminster Larger Catechism — Question 150—that some sins are more heinous than others due to the nature of the act or specific aggravations.
While the weight of our sin is staggering, the Gospel is even greater. We are indeed more sinful and flawed than we ever dared believe, but we are also more accepted and loved in Christ than we ever dared hope. It is precisely because we could not keep the law that Christ fulfilled it on our behalf. As Paul writes, 'But God commendeth his own love toward us, in that, while we were yet sinners, Christ died for us' (Romans 5:8). Our hope is not in our own performance, but in the finished work of our Savior.