Have you ever wondered why theologians use the term original sin? The term is often used to distinguish it from the actual sins and transgressions that flow from it. But in what way is it said to be original? Louis Berkhof (as usual) is helpful in dealing with this question. In his Systematic Theology, he writes:
This sin is called “original sin,” (1) because it is derived from the original root of the human race; (2) because it is present in the life of every individual from the time of his birth, and therefore cannot be regarded as the result of imitation; and (3) because it is the inward root of all of the actual sins that defile the life of man. We should guard against the mistake of thinking that the term in any way implies that the sin designated by it belongs to the original constitution of human nature, which would imply that God created man as a sinner. (p.244)
So Berkhof gives us three (3) reasons why we call it “original” sin. First, because this sin is “derived from the original root of the human race” (i.e. Adam). The sinful condition (including both guilt & the corruption of our whole nature) of all mankind is inherited from Adam and stems from his sin and fall in the garden (Genesis 3:1-24; Romans 5:12-21). Second, we call it “original” sin because this sin is “present in the life of every individual from the time of his birth.” In other words, in Adam we all come into this world as sinners; it is part of our nature, inherited from Adam. In the words of King David, “Behold, I was brought forth in iniquity, and in sin did my mother conceive me” (Psalm 51:5 ESV). He is not there saying that the act of his conception was sinful (e.g. adultery or fornication), but rather that he was a sinner from his very conception – it was a part of his fallen nature. Third, we call it original sin because it is the “inward root” of all of our actual sins and transgressions. It is cause & effect. So we are not sinners just because we sin; we sin because we are sinners by nature. The origin of our sins is to be found in Adam’s first sin and the sinful nature that we all inherit in him.
And note that the idea of original sin does not mean that the human race was originally created by God as sinful. We often say things like, “to err is human.” That may be so, but it is really only the case in Adam after the fall. Prior to the fall, “to err” was indeed possible, but it was in no way inherent in human nature as originally created by God.