For us to sin is to fall short of God's perfect Standard, His Ideal, which was perfectly exemplified in the life and death of Jesus, God's Anointed Prophet. To sin is to therefore fall short - an action that either fails to see the goal and therefore to ignore it entirely, or to seek the goal and fall short. Sinfulness, therefore is our "fallen shortedness," and is either our action or lack of action towards seeking the Goals Jesus says God wishes us to achieve.
Sin, therefore, is not a product of our Being, or a part of our Nature, but is an ACT on our part, or lack of Action, in relation to God's Ideal vision for what our lives can be.
Those who lack hope or who misunderstand God's Ideal and the achievement of it in Jesus' life and death and Message begin to despair, and when they despair, they misinterpret "sin" as something so deep within us that we cannot possibly achieve God's Ideals.
To believe this were the case, that sin is somehow a genetic fact that causes us to never reach God's Ideals (or, worse, makes us so that we can never even TRY to achieve them) would make God and His messenger, Jesus, into liars and convict them of gross injustice.
God requires moral perfection, but this is not an unreasonable requirement, nor an unreachable one. But if were UNABLE to do what God asks us to do , yet would still be punished for not accomplishing it, then that would make God both unrighteous and unjust. For to command us to achieve an Ideal, but then reveal that we cannot do it (or that it must be done FOR us by God, giving God the merit and us having nothing to do with the achievement of that goal) is unjust and immoral, if God truly judges us according to our Works – and we are assured that He will.
Happily, through the Good and Beneficial Message (Gospel) of Jesus, we know this isn't the case.
Jesus achieved the Ideals of God which he told us that we MUST seek to achieve, and that we COULD achieve. And because Jesus was human like us and walked the same earth as we did, breathed the same air, wept, slept, was hungry and was tempted, yet performed Good Works that pleased God and resisted temptation, we can be assured that those Ideals of God that Jesus preached to us ARE achievable by every human being.