Paul described what it means to be spiritually dead in Ephesians 2:1-3:
And you were dead in your trespasses and sins, in which you formerly walked according to the course of this world, according to the prince of the power of the air, of the spirit that is now working in the sons of disobedience. Among them we too all formerly lived in the lusts of our flesh, indulging the desires of the flesh and of the mind, and were by nature children of wrath, even as the rest (emphasis added).
Paul was obviously not referring to physical death because he was writing to physically alive people. Yet he said they were once “dead in their trespasses and sins.” It is sin that opens the door to spiritual death (see Rom. 5:12). Being spiritually dead means having a sinful nature in your spirit. Notice Paul said that they were “by nature children of wrath.”
In addition, being spiritually dead means possessing, in some sense, Satan’s very nature in your spirit. Paul said that those who are spiritually dead have the spirit of the “prince of the power of the air” working in them. The “prince of the power of the air” is no doubt the devil (see Eph. 6:12), and his spirit is working in all of those who are unsaved.
Jesus, speaking to unregenerate Jews, said,
You are of your father the devil, and you want to do the desires of your father. He was a murderer from the beginning, and does not stand in the truth because there is no truth in him. Whenever he speaks a lie, he speaks from his own nature; for he is a liar and the father of lies (John 8:44).
From a spiritual standpoint, those who are not born again not only have Satan’s nature residing in their spirits, but Satan is their spiritual father as well. They naturally act like the devil. They are murderers and liars.
Not all unsaved people have committed murder, but they are motivated by the same hatred as murderers, and they would murder if they could get away with it. The legalization of abortion in many countries proves that fact. Unsaved people will murder even their own unborn babies.
This is why a person must experience a spiritual rebirth. When one does, that sinful, satanic nature is removed from his spirit and is replaced with God’s holy nature. God’s Holy Spirit comes to reside in his spirit. He is no longer “spiritually dead” but is made “spiritually alive.” His spirit is no longer dead but alive unto God. Instead of being a spiritual child of Satan, he becomes a spiritual child of God.