How do we know that humans are evil by nature?

The doctrine of original sin explains that through Adam's disobedience, all humanity fell into sin, and thus we are born with a nature inclined towards evil. This is corroborated by passages such as Psalm 51:5, where David confesses, 'Behold, I was shapen in iniquity; and in sin did my mother conceive me.' This indicates that our sinful nature is not merely a result of our actions but is innate from birth. The context of the Old Testament kings, like Ahaz and his descendants, illustrates that even those appointed by God struggled with sin, reinforcing the universal truth of our fallen nature in need of divine grace.