What is spiritual death?

Death is separation. A physical death is the separation of the soul
from the body. Spiritual death, which is of greater significance, is
the separation of the soul from God. In Genesis 2:17, God tells Adam
that in the day he eats of the forbidden fruit he will "surely die."
Adam does fall, but his physical death does not occur immediately; God
must have had another type of death in mind—spiritual death. This
separation from God is exactly what we see in Genesis 3:8. When Adam
and Eve heard the voice of the Lord, they "hid themselves from the
presence of the LORD God." The fellowship had been broken. They were
spiritually dead.

When Jesus was hanging on the cross, He paid the price for us by dying
on our behalf. Even though He is God, He still had to suffer the agony
of a temporary separation from the Father due to the sin of the world
He was carrying on the cross. After three hours of supernatural
darkness, He cried, "My God, My God, why hast Thou forsaken Me?" (Mark
15:33-34). This spiritual separation from the Father was the result of
the Son's taking our sins upon Himself. That's the impact of sin. Sin
is the exact opposite of God, and God had to turn away from His own
Son at that point in time.

A man without Christ is spiritually dead. Paul describes it as "being
alienated from the life of God" in Ephesians 4:18. (To be separated
from life is the same as being dead.) The natural man, like Adam
hiding in the garden, is isolated from God. When we are born again,
the spiritual death is reversed. Before salvation, we are dead
(spiritually), but Jesus gives us life. "And you He made alive, who
were dead in trespasses and sins," (Ephesians 2:1 NKJV). "When you
were dead in your sins and in the uncircumcision of your sinful
nature, God made you alive with Christ. He forgave us all our sins"
(Colossians 2:13).

To illustrate, think of Jesus' raising of Lazarus in John 11. The
physically dead Lazarus could do nothing for himself. He was
unresponsive to all stimuli, oblivious to all life around him, beyond
all help or hope—except for the help of Christ who is "the
Resurrection and the Life" (John 11:25). At Christ's call, Lazarus was
filled with life, and he responded accordingly. In the same way, we
were spiritually dead, unable to save ourselves, powerless to perceive
the life of God—until Jesus called us to Himself. He "quickened" us;
"not because of righteous things we had done, but because of his
mercy" (Titus 3:5).

The book of Revelation speaks of a "second death," which is a final
(and eternal) separation from God. Only those who have never
experienced new life in Christ will partake of the second death
(Revelation 2:11; 20:6, 14; 21:8).