12 Therefore, just as sin came into the world through one man [Adam], and death through sin, and so death [both physical and spiritual] spread to all men because all sinned—
all … all
The word “all” indicates that sin is universalized just like death. The one man Adam versus the one man Jesus would lose its punch if “sinned” referred to personal sins. Adam acted as a representative of the whole race and so did Christ.
As Adam is our federal head, we inherit his transgression by imputation (accounting).
The concept of the sin of all men and the sin of Adam is the same. We were involved in some way in Adam’s sin; his sin was in some sense also our sin.
Adam is our natural head. There is a close connection between our physical and spiritual relationship to Adam. Both our spiritual and material natures come from Adam. In this sense we were present within Adam so that we all sinned in his act.
Though the one common nature that committed the “one offense” is divisible by propagation, the offence itself is not divisible, nor is the guilt of it. Consequently, one man is as guilty as another of the first sin, of the original act of falling from God. Adam was no more guilty of the whole sin than his descendants. His descendants are as guilty as he is.
Adam’s first sin is a common, not an individual, sin. God imputes this sin to the posterity of Adam. All men die because all men sinned. It is also true that mankind sins because people are separated from God; that is, they sin because they are dead.