From the moment Eve gained an imperfect and incomplete knowledge of good and evil, God has fought a steep uphill battle striving to convince every human of the horrors of sin. Sadly, He has often been unsuccessful.
In the first two chapters of Romans, Paul unequivocally and repeatedly addresses this question.
Is sin REALLY a big deal? YES! Because of God’s holiness. The wrath of God is revealed from heaven against all ungodliness and unrighteousness of men (Romans 1:18).
Is sin REALLY a big deal? YES! Because of sin’s inevitable digression. Suppression of truth in unrighteousness (Romans 1:18) is inexcusable (Romans 1:20); it darkens foolish hearts (Romans 1:21) which metastasizes into spiritual uncleanness (Romans 1:24-25), vile passions (Romans 1:26-27), and debased thinking (Romans 1:28-32).
Is sin REALLY a big deal? YES! Because of God’s righteous judgment. It is inescapable (Romans 2:3), personal (Romans 2:6), impartial (Romans 2:11), and fair (Romans 2:12-16).
Is sin REALLY a big deal? YES! And only a fool mocks at sin (Proverbs 14:9) and the certain wages it pays (Romans 6:23).