If I may, here's my attempt at explaining the doctrine Original Sin. Prior to the Fall, Adam and Eve were perfect and enjoy the grace of God. In the Book of Genesis, it is inferred that they know not the difference between good and evil. That is true. As St. Augustine would have argued, evil is merely the absence of good. How can one knows evil if everything that surrounds him is good? However, in their pride, they spurn God, disobeyed His command and to be led astray. This sin forever wounded human nature, since Adam and Eve deliberately reject His perfection. Through Adam and Eve, we inherit this imperfection, causing us, like Adam and Eve, to lose sight of God and His perfect goodness. Thus, all men were born in this deprived state through the first men.
Sacramental baptism, in Christianity, is not only an affirmation of faith, but is also a means for God's grace and mercy to work through. Since God is the essence of good and perfection, no "good" human deeds can possibly meet the requirement of God, for if He judge us, who would be sinless? But God is merciful, and instead of us striving to meet his standards, He comes to our level and restore us to His friendship. Through Baptism, God restored us to the state of grace, and wipe away the taint of the original sin. However, even with baptism, one still suffers from the impaired nature caused by the original sin. Thus, those baptized are still capable of committing sins against God, and loss that state of grace. But God, in His infinite mercy, is willing to extend His forgiveness again and again, despite our unworthiness. This is why the sacrament of reconciliation (confession) exists, in which the penitent receives God's forgiveness through a priest and making a firm resolution not to sin again.