How do we know that all men are under sin?

The Apostle Paul makes the case that all people, both Jew and Gentile, are under sin in Romans 3. He states that there is 'none righteous, no, not one,' which emphasizes the universal nature of sin. This doctrine is foundational in Reformed theology, underscoring that humanity, by nature, is sinful and in need of salvation. The truth laid out in Romans 5:18 reinforces this condition: 'Therefore by the offense of one, judgment came upon all men to condemnation.' Here, we see that everyone is born into sin and is guilty until made innocent through faith in Christ.