This page (by Craig Rusbult, copyright 2010) is an introductory overview, briefly summarizing the main ideas — about Bible-based evidence for Conditional Immortality — in my full-length paper where the ideas are explained in more depth. Later, beginning in 2014, I began writing a page that is better because it recognizes that Conditional Immortality would occur with either Annihilation (the only kind of Conditional Immortality described in this page) or Reconciliation. Here is my overview-summary from 2010:
Theologically, Conditional Immortality (CI) and Eternal Misery (EM) are almost identical. Both agree that we live-and-die, and later all humans will be resurrected to face judgment by God; those who are “saved” will live eternally in a joyful heaven with God; those who are not saved will suffer during judgment and in hell. The only difference is in the final state of unsaved humans: with CI their suffering (as in the "weeping and gnashing of teeth" that would occur when they recognize what they have lost, and regret it) is temporary until they pass into permanent non-existence; with EM their suffering lasts forever.
In Genesis 3 the sin of Adam produced three results: a decrease in quality of relationship with God, a decrease in quality of life, and a loss of immortality when God declared (Genesis 3:22) that sinners "must not be allowed to... live forever." When the full supernatural protection-against-death provided by God (symbolized by "the tree of life") was removed by God, Adam and Eve began to perish, with natural processes temporarily allowing life while gradually leading to eventual death, which God did not prevent with "the tree of life."
The concept of spiritual death — which, by ignoring Gen 3:22, claims that the penalty for sin was only a decrease of relationship with God — is defended by claiming that God said "in the day [yom] that you eat from it you will surely die" (Gen 2:17) and Adam did not die in that day. But ‘yom’ can mean either a 24-hour day or a longer period of time; with CI, during this period God did not prevent the gradual natural dying of Adam. But later, in heaven, God will prevent death by giving us back "the tree of life," as stated in Revelation 2:7 and 22:14.
We were created not with immortality, but for immortality that will be supplied by God; this immortality is conditional because it depends on the if-then conditions set by God; we must accept the grace offered by God through Christ, and live obediently by faith. We see a promise of eternal life in many places, including Revelation 2:7 & 22:14 (with "the tree of life") and 21:3-4 ("there will be no more death"), Luke 20:35-36 (we "can no longer die"), 1 Corinthians 15:42-57 (we will then be "imperishable... with immortality" because "death has been swallowed up in...victory through our Lord Jesus Christ"), 1 Tim 1:16 ("receive eternal life"). In each of these passages, and everywhere else in the Bible, human immortality is conditional; it is only for "him who overcomes, ... those who wash their robes, ... his people, ... those who are considered worthy, ... do the work of the Lord, ... believe on him."
In the beginning, sin led to a penalty of death in Genesis 3. In the Law, the penalty for serious crimes also was death (as in CI), not long-term imprisonment with suffering (as in EM). The OT ends with the death of evildoers by fire, so "not a root or a branch will be left... they will be ashes." In the NT teachings (by Jesus, Paul, James, John) the penalty for sin is death, and a common symbol is the total destruction that occurs when flammable material (weeds, chaff, branch...) is thrown into fire. The penalty is death, so with salvation God saves his people from death. God saved the son of Abraham from death by providing a substitutionary sacrifice. In the Passover, blood from a sacrificed lamb protected Hebrew sons from death. [and later a sacrificial system symbolically cleansed people from their sins, foreshadowing the central event of history when...] Most important, on the cross, when Jesus accepted the punishment for our sin, He died for us (He did not endure eternal misery) in the divine substitutionary atonement.
CI also seems more compatible with the character of God.
Probably most people (Christian and not) don’t think it is “justice” if humans are caused to suffer for an infinite time, as in EM, to punish them for sins committed during a finite time during their life on earth.
With both CI and EM, for a saved person the overall change is from nothing (before birth) to eternal joy. But for the unsaved there is a big difference: with CI the overall change is from nothing to nothing (in everlasting death) which seems fai r; but with EM the change is from nothing to everlasting misery, and it would seem justifiable for them to ask (as in Romans 9:20), "Why did you make me like this?" or even “Why did you make me at all, if you knew this would be my fate?”
Jesus described differing amounts of punishment in hell (Luke 12:47-48, Matt 11:24); with CI, two variables (intensity of suffering and its duration) can be adjusted, but with EM the only variable is intensity, if the eternal duration is identical for every unsaved human.
The concept of universal salvation — with God giving all unsaved people a second chance for salvation, and all eventually being saved — is emotionally appealing (I would join most people in voting YES for it if God asked us to decide) but the Bible tells us what God has decided, and it doesn’t seem to be universalism. Verses usually cited as support for universalism refer to “all”; with CI all living humans will love and obey God, with no sad feelings about the misery of former friends and family in hell, because those who rejected God are no longer living; but with EM there would never be a time when "all" will love and obey God, because the rejecting rebels would continue living forever [so the power of sin would be preserved forever].
Here are CI-explanations of Bible verses cited as support for EM:
Matthew 25:46 — the Greek word ‘kolasin’ is a noun, correctly translated “punishment” (in “eternal punishment”) instead of the verb “punishing”; an eternally lasting punishment-result (eternally lasting non-existence) does not require an eternally lasting punishing-process; the unsaved will be dead forever, and the saved will be alive forever.
Matthew 25:41 — "eternal fire" is impossible by natural process with EM (using an infinite supply of human fuel?) or CI, so eternal fire would be supernatural; or maybe ‘aionios’ (eternal) is used in one of its other potential meanings, to describe a fire ‘in the age to come’. Also, angels (including fallen angels?) "cannot die" (Luke 20:36) but unsaved humans are different, so in hell-fire they will be destroyed (as "weeds... burned in the fire", Matt 13:40) in "the second death", not kept alive.
Luke 16:19-31 cannot describe an intermediate state (they all have bodies before the resurrection) or the final state (the brothers are still alive) so it probably doesn’t describe any after-death state; instead, it teaches a lesson about living, and makes a prediction about unbelief.
Revelation 14:9-11 — it is only "the smoke" that rises forever, and "no rest day or night" declares the impossibility of a judicial pardon, so they will be tormented with no relief for as long as they exist. Isaiah 34 uses similar wording for symbolic ceremonial "smoke" rising forever from burned land that has not continued burning forever.
Revelation 20:10 — eternal tormenting of three creatures who are unique (coming from "the abyss", not killed in battle, not involved in The Final Judgment of humans) does not indicate that God will keep normal humans alive forever in "the second death" of Rev 20:14, 21:9.
The inertia of tradition and psychology of conformity make it easy to think like the majority, and difficult to think in other ways. But try to imagine that nobody believes in Eternal Misery, and you have just read a description of EM plus a summary of Bible-based arguments for and against it. Would you reject the accepted belief, CI, and replace it with a doctrine of EM proposing that God will keep unsaved humans alive forever so they can endure an endless eternity of misery in hell?
When defending a set of religious doctrines, if any part of the set is challenged it can be viewed as a challenge to other parts. This concern is warranted IF (and only if) there are logical links between parts, if the doctrine being challenged forms a logical foundation for other parts of the set, or is a logical outcome of other parts. This does not apply for CI and EM, because they are identical in all ways except one.
Christians should believe what the Bible teaches, so I encourage you to examine the Bible carefully before you reach a conclusion.