Romans #26 Adam's Fall, Part 2

Paul interrupts the comparison that he intends to make—and which he will finish in verse 18—by adding what we might call a parenthetical thought in vv. 13-14. He does this in order to elaborate on the point of v. 12 that sin and death are universal—everyone in Adam's race experiences them both.
Adam, the first man, was the head of the human race. He was our representative—our covenant or "federal" head. God made a covenant with Adam in which if Adam obeyed, then he—and all of the people he represented—would live in righteousness forever.
But if Adam disobeyed God, he, together will all of those whom he represented, would be plunged into sin and death. Sadly, as Genesis records, Adam sinned and, as we teach our children, "In Adam's fall, we sinned all."
Because he was our representative, our covenant head, we sinned in him. That is the point of verse 12.
Adam's sin resulted in the fall of the human race. Sin and death are now the universal experience of all of the people whom he represented before God. We are implicated in his disobedience. His sin, guilt, and condemnation are imputed—credited—to us because we are by nature united to him.

This point—our union with Adam by virtue of his federal headship of the human race—is so important to the comparison Paul makes between Adam and Christ in 5:12-18, that he interrupts his flow of thought in v. 12 and inserts vv. 13-14. In those two verses he underscores our union with Adam in sin and death by showing that death reigned over the human race even before the law was revealed through Moses on Mt. Sinai.