The first and primary reason God punishes sin is in order to prove himself righteous and just. A second reason God punishes sin is to bring back an erring son or deter others from sinning. Spiritual death, physical death, and eternal death are all punishments for sin, as are certain sufferings in this life. But beyond question, eternal death is the gravest punishment for sin imaginable. In this case, God makes it impossible for the sinner who dies apart from the saving mercies of Christ to ever be reconciled with Him.
Sin always has consequences for both the present life as well as the next.
The Christian cannot escape certain consequences of sin in this life or judgment for sin in the next, but this judgment does not revoke his salvation. I am not here arguing for some kind of Platonic or Gnostic dualism, or anything like it. I am simply saying, as C. S. Lewis had occasion to say, that we were made to live in two worlds simultaneously. All rights reserved.