The Greek term "hamartia" which is translated as "sin" in English becomes "peccare" or "errare" in Latin. In antiquity, it still had its etymological sense of "to stray" or "to wander from a path"; it does not necessary imply deliberate evil. For the Stoics, to err meant to depart from the natural order. Because the Stoics were generally fatalists, emphasizing necessity, erring was a mental condition, occasioned by ignorance or willfulness. They did not really have a major sense of "sin" but were concerned with how to attain virtue through a life of calm and reason.
For Augustine, sun was a condition of fallen humanity, due to the imperfection of the flesh. A major element of his notion of sin was a concept of evil as the absence of good rather than a positive quality. Thus humans, separated from God by the fall, are sinful because they lack perfect good.