Don't be afraid of those who kill the body, but are not able to kill the soul. Rather, fear him who is able to destroy both soul and body in Gehenna. -- Matthew 10:28, World English.
Jesus is not here saying that the soul cannot die, as he goes on to say: "but fear him who can destroy both soul and body in Gehenna," using the fiery valley outside the walls of Jerusalem as a symbol of destruction. To have this mean eternal torture while one's body is dead, one has to change "destroy" to "torture", and also change body to "spirit" or something else.
Jesus is certainly not contradicting the entire body of scripture in this regard, which over and over states that the soul dies, can be destroyed, speaks of dead souls, etc. The body is not the soul, but it is a component of the soul. The soul is made up the body and the spirit (or breath) of life from God. (Genesis 2:7) When one dies the soul dies [ceases to be a living sentiency] and the original life process is reversed. (Ecclesiastes 12:7) With the life-giving source departed from the body, the soul [sentiency] ceases to exist.
For those who are not familiar with the Biblical teaching that the soul dies and is not immortal, we give the following: Using the King James Version with marginal references: The soul dies: Job 36:14 (margin); Psalm 56:13; 78:50; 116:8; Ezekiel 18:4, 20; James 5:20. The dead soul is not alive: Psalm 22:29; 30:3; 33:18, 19; Isaiah 55:3; Ezekiel 13:19; 18:27. The dead soul ceases: Psalm 49:8. The wicked soul is destroyed: Psalm 35:17; 40:14; Proverbs 6:32; Ezekiel 22:27; Matthew 10:28; Acts 3:23; James 4:12. The wicked soul is consumed: Isaiah 10:18. The wicked soul is devoured: Ezekiel 22:25. The wicked soul perishes: Matthew 16:25, 26 (the Greek word for soul is here often translated as "life"). The wicked soul is cut off: Leviticus 22:3; Numbers 15:30. The soul of the saints are pictured as being slain as in sacrifice, as though ashes under the altar, are asleep in death, waiting for the resurrection. -- Revelation 6:9,11.
There are many more scriptures in the Hebrew that show that the soul is not immortal; this cannot be seen in most translations, however, since the word for soul is often substituted by "creature", "body", etc. On the other hand, there is not one scripture in the entire Bible says anything about the inherit immortality of the soul, or that the soul continues to live after the body dies.
However, getting back to Jesus' statement: The above scripture does not apply to everyone, but only to the believers. Those who put faith in Jesus are counted as alive in God's sight in view of the resurrection [not immortal soul] promises. This is shown in Luke 20:34-38, where in speaking of the resurrection, Jesus says: "For he is not the God of the dead but of the living, for all live to him." -- See also: Romans 4:5,22-25; 5:1,18; 6:11.
Further, and again speaking of the day when the righteous will be raised, Jesus said (John 6:54): "Whoever eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day." If a believer, after having this life reckoned to him through faith in the blood of Jesus, and partaking of the holy spirit -- powers of the age to come, turns away from this faith, he will lose the life he had obtained through that faith, for there is no longer any more sacrifice for sins left. (Hebrews 6:4-6; 10:26) Such a person would go into the second death, for which there is no ransom sacrifice provided. Thus Jesus, in speaking to his disciples, said the above statement as a warning. No human can take away our life -- soul -- that has been made alive in God's sight. Only God himself can do this by destroying our soul as new creatures.
Something else we might note here: Gehenna is not a symbol of the eternal roasting theory, for Jesus speaks not only of the soul being thrown into this valley, but also of fleshly bodies. In a different setting Jesus speaks similarly of the fleshly body being thrown into the fires of Gehenna. -- Matthew 5:29,30.
Some try to say that the destruction of the body is physical death, but the destruction of the soul is spiritual death. It should be apparent, however, that if "kill" in respects to the body means actual destruction -- ceasing to exist, then also "kill" and "destroy" used in respects to the soul being spoken of also means actual destruction -- ceasing to exist. We have yet to see an explanation of exactly how the concept of "spiritual death" as held by dualists would fit the death of the soul in Gehenna, since it is usually claimed that mankind fell under the sentence of spiritual death in Adam.
It has also been claimed that the death of the body is the first death (physical death), and that the death of the soul is the second death (spiritual death). This raises the question as to whether mankind has received the condemnation of the second death from Adam. Was mankind already under a condemnation of physical death (the alleged first death) before Adam sinned, so that, when Adam sinned a second death was attached to the condemnation already upon man? There is nothing in the scriptures about such.
Scripturally, the first death is the death in Adam, for which reason Bible Students usually refer to this death as "Adamic death," which death Jesus provided a ransom for. (Romans 5:12-19; 1 Corinthians 15:21,22; 1 Timothy 2:5,6) The second death actually belongs to the age to come, when those who, after receiving the full knowledge of truth, willfully desire sin will be thrown into the lake of fire. (Revelation 20:11-15) There is no more ransom, so this death will mean eternal destruction, just as it means eternal destruction for the Adamic death and hades, which are also thrown into the lake of fire. However, in this age, there are those who become sons of God who are reckoned alive and who also partake the powers of the age to come. (Hebrews 6:4,5) If these, after having been sanctified by the blood of the convenant, willfully practice sin, they have have no more sacrifice for sins. (Hebrews 10:26-29) They had already used up, so to speak, the merit of Jesus' sacrifice on their behalf. They cannot return to the condemnation in Adam, thus they come under a new condemnation, a more severe punishment, the condemnation of the age to come, that is, the second death.