It is important to understand what the phrase "original sin" does not mean as it is to understand what it means. It does not mean that that sin belongs to the human nature as God created him, nor that sinfulness attaches to reproduction and birth. In evangelical theology, it does mean that 1) every person is marked by sinfulness from birth, 2) it is there already before a person commits an actual sin, and 3) it is the root and source of all actual sins, and 4) it derives from Adam, the representative (or federal) head of the human race before God. Historically, Augustine was the first theologian to give the doctrine a clear and concrete formulation, for whom original sin is both "inherited corruption" and "inherited guilt."
The idea of 'original sin' has nothing to do with the origin of sin per se, but with the question of how we became sinners. Traditionally, the doctrine of original sin is closely weaved into, and viewed as an aspect of, the concept of the Fall. That Adam and Eve, and their disobedience in the consumption of the fruit of the tree of the knowledge of good and evil, is seen as the fountainhead of our human plight as sinners; "For just as through the disobedience of the one man the many were made sinners, so also through the obedience of the one man the many will be made righteous" (Rom 5:19). The vital question is how exactly this is to be understood, to which two main schools of thoughts—or a hybrid of them—have held sway in the history of the Church: 1) known as the Reformed theory, the Federal theory, or Representative theory, this view essentially holds that Adam was the legal representative of the human race in his covenant with God; his failure, therefore, laid on us who are from his line (except Christ who is the Incarnate Son of God) are similarly accountable for the penalty of his sin, 2) the Augustinian or Realist theory (so-called because Augustine of Hippo was the one who developed it) that views Adam's sin is passed on to us almost as if it has become part of our genetic composition; a common illustration is that of an acorn (the 'seed' of the oak tree) which simply manifests what it has inherited from its parent tree. Like all interpretive attempts both views have their strengths and weaknesses.
For a more detailed overview, see Lecture 28: Original Sin of Systematic Theology 1, by Dr Bruce Ware. Biblical Training. N html
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