Death, both physical and spiritual, share this in common: that they involve separation and DO NOT represent the cessation of existence.
In physical death, the soul separates from the body, the body decays and the dead person’s “dust returns to the ground it came from” (Ecclesiastes 12:7, NIV).
Likewise, spiritual death involves separation, and in particular, separation between a person and God.
Isaiah 59:2 (NIV) reads: “But your iniquities have separated you from your God; your sins have hidden his face from you, so that he will not hear.” (emphasis mine), and note how in Ephesians 2:1, this separation is translated as death: “As for you, you were dead in your transgressions and sins”. (emphasis mine)
This concept is again reflected in Colossians 2:13 (NLT; with emphasis mine):
You were dead because of your sins and because your sinful nature was not yet cut away. Then God made you alive with Christ, for he forgave all our sins.
Bearing this in mind, have you ever wondered what the Lord Jesus Christ meant when he said to “let the dead bury their own dead”? (Luke 9:60). It is obvious that a physically-dead person cannot bury another physically-dead person, however, spiritually-dead people are capable of burying a physically-dead person. Thus, the New Living Translation (NLT) is accurate in its translation of Luke 9:60 as follows (with emphasis mine):
But Jesus told him, “Let the spiritually dead bury their own dead! Your duty is to go and preach about the Kingdom of God.”
Finally, it is important for believers to note that there are things that can lead to spiritual death and that we are to avoid them (Romans 6:20-21; NIV; with emphasis mine), lest they cause us to die spiritually again:
20 When you were slaves to sin, you were free from the control of righteousness. 21 What benefit did you reap at that time from the things you are now ashamed of? Those things result in death!