"Therefore, just as through one man sin entered into the world, and death through sin; and thus death spread to all men, because all sinned." (Roman 5:12). The picture is as if Death is standing outside a closed door and man's sin is throwing the door open and allowing Death to come in to the human. This is not Physical Death, but it does have reference to another kind of Death which Pauls speaks about constrasting Spiritual Death and Spiritual Life.
Spiritual death is the Nature of the Devil and Spiritual Life is the Nature of God; out of Spiritual Death come hatred, jealousy, murder, and out of the Nature of God come love, forgiveness and peace. We can understand the prevalence and power of sin in the world today. We many legislate all we please, but until we change the Nature of Man, sin will grow and flourish; we may stop it here or there, but it will spring up elsewhere. The only hope of the human is a new Nature. Spiritual Death, then, is the Nature that lies behind all sinned committed. Man commits Sin, because his Nature produces that kind of conduct. Spiritual Death made Man mortal, only Spiritual Life can make him immortal.