Why do Christians believe that all are born into sin?

In Romans 5:12, it is explained that through one man, sin entered the world, and death through sin; thus, death spread to all men because all sinned. This means that every individual is born with a sinful nature, akin to the lame beggar in Acts 3, who was helpless from birth. This doctrine of original sin is foundational in Reformed theology, positing that all of humanity requires the redemptive work of Christ to be made whole and saved from judgment. Without acknowledging our innate sinfulness, the need for a savior cannot be truly appreciated.