Who does a person serve if they are a child of Adam?
If a person is a child of Adam, they serve Adam—the old Adam—and remain under his control. The contrast is obedience to God's gracious invitation, which brings righteousness and right standing with God now and for all eternity, rather than spiritual death.
Les explains that a person is a servant to whomever they give obedience. If a person is a child of Adam, that is the one they serve; they are under the control of Adam, the old Adam. That path brings spiritual death. On the other side, one can be obedient to God's invitation and grace and enjoy righteousness and right standing with God now and for all eternity. Les stresses it is either/or, the only choice in life. No human works, church memberships, baptisms, sacraments, or elements can put old Adam to death; only the work of the Creator God Himself can do what Paul teaches in justification by faith.