Ian McFarland on original sin
In short, the claim that human beings are born sinners (i.e., actually guilty of sin and not just predisposed to sin) derives from and depends logically on the more fundamental conviction that the ultimate vindication of particular human lives before God is pure gift.
This soteriological framework affects Christian claims about what makes human beings human. If humanity’s ultimate destiny as human before God is a matter of gift, it follows that the source and guarantor of human identity is located outside the self in God. This perspective is inconsistent with the penchant of most modern thinkers to see the (free) exercise of the will as the source of human identity. In other words, it is a corollary of the Augustinian doctrine of original sin that human identity is rooted in a reality that is prior to human willing. This reality is either the grace of Christ, which leads to eternal life, or the taint of original sin, which leads to death and damnation.
This way of thinking has the further effect of transforming sin form a moral category defined in terms of discreet acts of the will to an ontological one that describes the conditions under which willings takes place. The will is directed by desire, which is in itself not under the control of the will. Again, the will does whatever it wants, but it cannot want whatever it wants. Because in the aftermath of the fall human desire is turned away from God, the will is constitutionally sinful prior to any act of the will (i.e., prior to any concrete thought or deed that could be subjected to moral assessment). […] In short, while a sinner is one who commits sin, from an Augustinian perspective it is the fact of being a sinner that leads to concrete acts of sinning and not the other way round.
from “The Fall and Sin,” in The Oxford Handbook of Systematic Theology, (OUP, 2007), 148. [emphases added]