Sin entered the world through Adam, and we know this because all men sin. Our flawed nature is a consequence of that sin. Sin existed before the law, and although sin is not counted where there is no Law, that does not mean that we do not have any guilt for that sin. Indeed, the Law came to increase the trespass of the sin that already existed. Cain, for example, remained culpable for his actions and the Flood came as a consequence for sin.
Some may not recognize that their sin is sin against God, and those people do not have the same sense of sin as Adam, but they are culpable nonetheless. Adam's sin was especially heinous, since he knew in a very unique way what he was doing through that sin. Yet sin continued through people who did not know sin in the same way.
Jesus is also one man, but instead of condemnation, this new man brings the grace of God. By one man's disobedience, all men became sinners, but now, through one man's obedience, all men become righteous. Even so, we must be at peace with God to accept the grace that God is offering us.
The closing theme is Gerard Satamian's Chansons Sans Paroles Op. 2 Pastorale, from the album Dry Fig Trees. www.magnatune.com