5:12 just as sin came into. Paul here begins a comparison that is not concluded until vv. 18–21. The comparison is interrupted by a meditation extending through v. 17.

through one man. Death is not natural to humanity, but is the direct result of sin (Gen. 2:17).

because all sinned. The universal reign of death is the consequence of sin. Paul does not explain how all mankind was involved with Adam in his sinning, but simply asserts the fact. All sinned in the sin of Adam. See “Original Sin and Total Depravity” at Ps. 51:5.