How do we know that humans are born in sin?

The doctrine of original sin is foundational in Reformed theology, as indicated in Romans 5:12, which states that sin entered the world through one man, Adam, and death spread to all men because all have sinned. This theological concept explains the inherent sinful nature of humanity, beginning at birth, underscoring that every individual is born with a propensity to sin. Additionally, verses like Psalm 51:5 affirm this reality, indicating that humans are conceived in iniquity, which is foundational for understanding the necessity of salvation through Christ.