12 Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because all sinned—
through [emphatic instrumental case] one man [Adam],
The “one man” here is Adam, and his decision to sin plunged the human race into sin. This decision changed his essential constitution from innocence to sinfulness. His action was a completed action in past time at one point in history. This ended the age of innocence.
Adam’s sin completed the introduction of sin into the world. The complete institution of sin into the world introduced spiritual death into mankind—separation from God. The words “through the one man” point to Adam as the instrument by which sin entered the world. Adam’s sin was the basis of universal sin and universal death.
The use of the word “one” stresses the unity of the federal head with those under Adam’s authority. The word “one” occurs 14 times in verses 12 to 21.
Mankind inherits the proclivity to sin from Adam.
Adam’s sin constituted a constitutional change in all mankind. God construes the one trespass of Adam as the sin of all. All mankind was in the loins of Adam when he sinned. Adam’s original sin was the result of his representation as the federal headship of humanity.
The genetic outcome results in everyone inheriting the sin capacity. The Bible uses such terms as “flesh” and “body of sin” to describe this development.
On the federal representative side, God imputes Adam’s guilt to us. This is the legal guilt we receive from Adam.