Entire books on this subject have been written, as well as innumerable articles, with both sides claiming Bible verses to support their interpretation. And while this topic will likely be debated until Jesus returns, it is one that I feel I cannot be silent on as the very character of God is at stake. Please note that my goal was to succinctly expound on the history of the doctrines, and how the developed, thus I have not covered every bit of evidence that exists, nor addressed every verse in the Bible that touches on this sensitive subject.
A: Judaism has always held to annihilation for the wicked (kiluy neshama).
#1 – Old Testament
God: The soul that sins, it shall die. If a man is righteous and does what is just and right, he shall surely live. If he has a son who is violent (list of other evil deeds), he shall not live. But if a wicked person turns away from his sins, and does what is just and right, he shall surely live (Ezekiel 18).
Obviously, this is not talking about the physical death of the body, as everyone, including the righteous, die physically.
#2 – The Talmud (commentary on Judasim written by Orthodox Jews)
Rosh Hashanah 17a: The bodies of those liable to [the penalty of] being "cut off" cease to exist. That is, the body's strength or animal power ceases, and "their souls are burnt up."
#3 – New Testament
Jesus (a Jew): For God so loved the world, that He gave his only begotten son, that whosoever believes in him, will not perish, but have eternal life (John 3:16). Jesus states that immortality (eternal life) is conditional on believing in Himself. According to Strong’s Concordance, the definition of perish is: "destroy, put an end to, kill."
Jesus: Do not fear those who can kill the body, but fear Him who can destroy both body and soul in Gehenna (Matthew 10:28). If the Jews were wrong about believing in annihilation, then Jesus did a horrible job of trying to get them to see the error of their ways.
Paul (a Jew): Seek for glory and honour and immortality. Fight the good fight of faith, lay hold on eternal life. (Rom 2:7 and 1 Timothy 6:12). Paul, a Jew, held to conditional immortality. If we are inherently immortal, there is no reason to seek it, or lay hold of it.
Paul also believed the wicked would be destroyed: those who disobey God “shall be punished with everlasting destruction from the presence of the Lord.” (2 Thessalonians 1:9) According to Strong’s Concordance, the definition of destruction is: "ruin, destroy, death."
B. The Greeks believed in the inherent imperishability of the soul and eternal punishment.
#1 – Greek Philosophy: One of the main themes in the the Greek play, Phaedo (also known as Plato's On The Soul), is the idea that the soul is imperishable. This play was written to combat another Grecian theory, that of naturalism (belief that the soul dies with the body).
#2 – Greek Religion: Hades, god of the underworld, tortured the souls of the wicked in fiery chambers. (This blend of the Greek religion with Gentile Christianity is where the false notion arises of Satan ruling from hell/Gehenna, when in fact, Gehenna was created for Satan, and you don’t give the key to the jail to the highest maximum security prisoner).
C: Lost in translation: The Greek "anionios" translated to the English "eternal."
The problem we see today lies in the modern definition/understanding of “eternal / everlasting / forever” which is: without beginning or end. But the original Hebrew and Greek definition of “eternal / everlasting / forever” is “without ceasing until the end.” There is much proof of this:
#1 – Many Ancient Greek scrolls contain numerous examples of Roman emperors being described as aionios, the Greek word translated “eternal” in English Bibles. But all that is meant is that they held their office for life -- not that the emperor was immortal, or that his reign never ended.
#2 – Dead Sea Scrolls: the wicked will suffer “unending dread and shame without end, and of disgrace of destruction by fire of the region of darkness. And all their time from age to age are in most sorrowful chagrin and bitterest misfortune, in calamities of darkness till they are destroyed with none of them surviving or escaping” (1QS 4.11-14). Note that this is saying punishment in Gehenna is without end UNTIL they are destroyed. The Dead Sea Scrolls prove the historic definition of words such as perish, die, and destroy as well as eternal, everlasting, and forever and validate the Bible's use of them without ANY contradition. Only when one forces a modern definition onto the words does the Bible seem to contradict itself and/or force the reader to choose a symbolic definition for certain words (i.e. perish means eternal life in an unhappy place).
It should be noted that Jesus' use of the words kill, perish, and destroy, and Paul’s use of the word destruction (as well as both of their stressing conditional immortality) demonstrates that the Greco-Roman belief in the inherent immortality/imperisahbility of the soul was the inaccurate view, not the Jewish belief in conditional immortality and annihilism.
D: Combating Universalism.
So how did the early church, which believed in conditional immortality and annihilation, turn into a church that held to inherent immortality and eternal torture? What influenced the change to the ancient defintions of the above mentioned words?
The early church was plagued with Gnosticism for centuries (John’s gospel and his three epistles were written against Gnosticism). Gnostics denied the resurrection, and many held to universalism (the belief that everyone eventually goes to heaven). As universalism started to spread (and even adopted by some Church fathers such as Origen), other church fathers started writing against this heresy, and accurately stressed that the punishment does not end (it is final, there is no coming back; you go into Gehenna, you do not come out; there is no end to the punishment in the sense that God does not eventually let everyone go to heaven).
But...since the Church Fathers (and all the newly-converted Gentile Christians) came from a Greco-Roman background, both philosophically (in that they believed in the inherent immortality of the soul) and religiously (they believed the souls of the wicked were tortured), it is quite easy to see how the Jewish concepts of conditional immortality and annihilism came to be lost in ony a few centuries. And by the time Universalism was finally condemned several centuries later, eternal torment was the predominant belief of the church.
The Bible's use of “eternal / everlasting / forever” denotes the finality of the punishment of sin, not the duration. The duration will likely be based on the particular individual as Jesus said the inhabitants of Sodom and Gomorrah will fare better than Capernaum (Matthew 10:15), and elsewhere that some would receive few “blows” and others “many” blows (Luke 12:48). And regardless of how long it takes the fires of Gehenna to burn one's soul into nothing, there will be wailing and gnashing of teeth.
The Bible’s stress on the finality aspect of this punishment clearly denies the validity of universalism, and for good reason. Universalism is probably the most dangerous doctrine that could ever be purported, and if it had prevailed in the early church, it would have been the death of true Christianity. But the doctrine of eternal torment, originating in paganism, also poses a threat to Christianity as it slanders the very character of God, and makes Him into a moral monster.