From this single episode in mankind’s history, we learn that all mankind was condemned to die because of Adam’s sin. Because of Adam’s sin, every descendant of Adam and Eve comes into existence under the power and “reign” of death. Paul makes this fact clear in Romans 5:12-14: “Therefore, even as through one man sin entered into the world, and through sin death, and thus death passed through into all mankind, on which all sinned -- for until law sin was in the world, yet sin is not being taken into account when there is no law; nevertheless death reigns from Adam unto Moses, over those also who do not sin in the likeness of the transgression of Adam, who is a type of Him Who is about to be.”
I believe the problem is resolved when we understand Paul to have been using a figure of speech known as “prolepsis” here. According to this figure of speech, something that is future is spoken of as though it has already taken place (or as if it were already present) in order to emphasize the certainty of its taking place. At this present time, the world is not conciliated to God (for all mankind has not yet been constituted just). Most people remain under condemnation, and are (as all believers once were) "estranged [from God] and enemies in comprehension, by wicked acts" (Col. 1:21). What was accomplished prospectively through Christ’s death for our sins has not yet been fully realized and applied to all mankind. But it shall be, and that is what makes Paul’s evangel good news for all.
“For if many died through one man’s trespass...” (v. 15)
“Therefore, as one trespass led to condemnation for all mankind...” (v. 18)
Adam is “the one” whose act of disobedience negatively affected those whom he represented (“the many”). This category of persons is also referred to as “all mankind” in v. 18. In contrast to Adam, Christ is “the one” whose act of obedience positively affects the same “many” who were negatively affected by Adam’s sin. These are also referred to as “all mankind” in v. 18.