R. L. Dabney writes:
If you were a soldier and had not deserted the colors of your country, an accepted substitute would free you from all service and punishment. You are guilty of desertion toward God, and are also bound to pay a service to which sin utterly disables you through your own folly and fault. From these obligations Christ frees you, but it is only to bind you to His service more firmly by love. Now you should follow the Captain of your salvation with all your might, longing to follow Him better, not from fear of being shot for desertion (that danger is gone if Christ died for us), nor from fear of losing emoluments (they are already earned for us by our Substitute, and paid in advance to true believers), but because He asks us to follow Him. And now, if we love Him, we would die for Him were it necessary, because He died for us. If we do not love Him, it is proof that He never became our Substitute. “Now are ye My friends, if ye do what-soever I command you.” John xv.14.
(see the entire tract here)