Death is not a punishment for Christians (Rom 8:1). Christ paid the penalty for our sins; the death of a believer is not punishment from God or the result of a penalty due us for our sin; in fact, God views the death of a believer as “precious” (Ps 116:15). Paul views the early death of some believers who had been abusing the Lord’s Supper in the church at Corinth (1 Cor 11:30) as a disciplining or chastening process from God, and not as a result of condemnation (1 Cor 11:32). (We define punishment as retribution from God intended to do us harm, and discipline as hardship through which God intends to do us good.)
We know the penalty for our sin is death, both physical and spiritual (Rom 6:23). But, Christ completely and fully paid the penalty for our sin (John 19:30; Matt 27:51; Heb 9: 11 - 10:18).
Death is the final outcome of living in a fallen world. In His infinite wisdom, God chooses to gradually apply the benefits of salvation to us over time, rather than all at once (ie, immediate justification at the moment of conversion (Rom 3:21-22), progressive sanctification through the lifetime of the believer (2 Cor 3:18), ultimate glorification at the resurrection (1 Cor 15:52-53), etc.). Similarly, He has not chosen to immediately remove all evil from the world that we live in - but to wait until the final judgment (Rev 20:11-15)and the subsequent establishment of a new heaven and a new earth (Rev ch. 21). Thus, we must recognize that we live in a fallen world suffering the effects of sin (Rom 8:19-22), and our experience of salvation is still incomplete.
Scripture indicates the last (final) aspect of this fallen world to be removed is death (1 Cor 15:26; 15:54-55). Until this time, death remains a reality even in the lives of Christians. So, believers suffer death as a consequence of living in a fallen world, and not as the penalty due our sin. Related to the experience of death are other results of the fall that can potentially harm our physical bodies and signal the presence of death; thus Christians as well as non-Christians experience: aging, illness, injury, natural disaster, accidents, etc, Although God often answers prayer to deliver believers (and also non-believers) from some of these effects for a time, nevertheless, Christians eventually experience all these things to some measure. Until Christ returns, all of us will grow old and die/ God, in His wisdom, had chosen to allow us to experience death before we gain all the benefits of salvation that have been earned for us
God uses the experience of death to complete our “sanctification”. We can view aging, weakness and sometimes sickness leading to death as another kind of discipline that God permits us to experience that our process of sanctification may be furthered. Our sanctification (becoming more and more like Christ - Rom 8:29) is ultimately completed at the moment of our death when we pass into the presence of God (2 Cor 3:18) and we are challenged to remain faithful till death (Rev 2:10) and honor God even in death (Phil 1:20)