Arguably one of the most central doctrines of Christianity is the belief in what is called Original Sin. That is the belief that human beings are all born in sin and are sinful by nature. The reason for this goes back to the sin of Adam and Eve in the Garden of Eden when they ate of the fruit of the Tree of Knowledge of Good and Evil.
After the first humans ate of the fruit they and all of their offspring would inherit this sinful state and this provides for the rationale to the problem that Jesus would serve as its only solution according to Christianity. Through accepting Jesus and undergoing baptism one can be cleansed of that sin.