The Reformed objection to Dispensationalism’s carnal-Christian theory is based on the Reformed doctrine of regeneration. What is generated anew is the person’s nature. The heart of the sinner is truly changed. Once in bondage to sin, the sinner is now liberated unto newness of life. The fruit of obedience is both inevitable and necessary; it is immediate. Obedience is by no means perfect, nor does it in any way contribute to the ground of one’s justification. Its absence, however, points to the absence of regeneration. A totally carnal person is an unregenerate person, and an unregenerate person is an unsaved person.
Sproul, R.C.; Grace Unknown: The Heart of Reformed Theology; Baker Books; Grand Rapids, MI; copyright 1997; p. 193