What is sin whose sins we are guilty of? A good way to delineate it is to describe its effects. Paul explained it saying "if I am doing the very thing I do not want, I am no longer the one doing it, but sin that dwells in me" (Ro 7:20). He said that we indulge "the desires of the flesh and of the mind, and were by nature children of wrath" (Eph 2:3). The source or reality of it is in man's nature. David said, "I was brought forth in iniquity, and in sin my mother conceived me" (Ps 51:5). A more direct revelation from Paul is that it is "the principle that evil is present in me" (Ro 7:21). "Sin [evil] was in the world" (5:13).
Many people define the original sin as being "through the one man's [Adam's] disobedience the many were made sinners" (Ro 5:19). "Death spread to all men, because all sinned" (:12). That sin is explained in Romans 5:18 by the NASB as "through one transgression", the NIV "as the result of one trespass", and the NKJV "by the offence of one." An offence is a violation involving a breaking of a law or rule as with "the offence of Adam" (:14). It is "iniquity . . . in sin" (Ps 51:5) as "disobedience" (Ro 5:19). That "the many were made sinners" (:19) is explained by the Greek word kathistemi meaning that one is appointed to a position designating that he hasn't specifically qualified himself for it. That is why you received the guilty status even though you "had not sinned in the likeness" (:14) of Adam.
Consequently "there resulted condemnation of all men" (Ro 5:18 NASB) and "judgment came upon all men to condemnation" (:18 NKJV). Condemnation means that a penalty has been assessed as a judgment in that Adam was told he would "surely die" (Ge 2:17) which is equivalent to a death penalty. "The Lord God commanded the man, saying . . . 'from the tree of the knowledge of good and evil you shall not eat, for in the day that you eat from it you will surely die'" (:17). However, the serpent tempted Eve asking, "'Did God really say, 'You must not eat from any tree in the garden''?" (3:1). How did he know this? He was there "from the beginning" (Jn 8:44). Then he lied saying, "'You will not surely die'" (Ge 3:4). Jesus said, "'Whenever he [the devil] speaks a lie, he speaks from his own nature, for he is a liar and the father of lies'" (Jn 8:44).