According to the Holy Bible, spiritual death refers to the soul’s separation from God. Thus, unlike physical death, which is a separation of body and soul, spiritual death refers to separation of man and God.
It happens due to sins and our own disobedience to His gospel. For instance, when Adam sinned against God by eating the forbidden fruit, he died spiritually but continued to live physically fro numerous years.
Thus, it is believed that we are all born spiritually dead, or in other words, spiritually separated from God.