Mankind has always been fascinated by eternity, so what is Christian immortality or Conditionalism?
God has put eternity in our hearts or given us thoughts of immortality. We are finite creatures, but by nature, we desire to live on…and on. The God-given fight-for-survival instinct makes us fight for our lives. Animals have the same desire to survive. God “has made everything beautiful in its time. Also, he has put eternity into man’s heart, yet so that he cannot find out what God has done from the beginning to the end” (Eccl 3:11). We’d like to think that we know the ending, but it’s not humanly possible to know the end of our days. God says our “days are determined, and the number of his months is with you, and you have appointed his limits that he cannot pass” (Job 14:5), and “in your book were written, every one of them, the days that were formed for me, when as yet there was none of them” (Psalm 139:16). James even says “you do not know what tomorrow will bring. What is your life? For you are a mist that appears for a little time and then vanishes” (James 4:14), so even though we might yearn for immortality or eternity, we need to know where we will spend it at.
Conditionalism or conditional immortality is the concept that when a person believes in Jesus Christ, they become attached to immortality. Jesus Himself is the life, the truth, and the way (John 14:6), and there is no other way (Acts 4:12), so our eternal life in the kingdom depends on our having a relationship with Jesus Christ. If God has brought us to repentance, and we put our trust in Christ, we have received eternal life, or immortality, but those who reject Christ will also live on forever. It’s only that their state will be beyond description. As Lazarus and the Rich Man showed, the rich man will remember his past life, have eternal regrets, will still feel thirst, and pain, and heartbreak, but he was not be able to move from where he is to those in the kingdom, nor to go back to his family (Luke 16:19-31). Both those who believe in Jesus and those who reject Jesus still live on, so in that sense, those in hell will be immortal as the human soul is immortal…even after death, but their separation from God was conditional. They rejected their only hope to be saved. Their condition is sealed after death when their judgment comes (Heb 9:27). The human body is mortal, but the soul is immortal. There was a time when we did not exist, but there will never be a time when we do not exist. Wherever we were in our relationship with Christ after death or when Christ returns, is conditional upon our receiving or rejecting Jesus Christ. Speaking of such a place, Jesus said “will say to those on his left, ‘Depart from me, you cursed, into the eternal fire prepared for the devil and his angels” (Matt 25:41), and where is this place? It is “where their worm does not die and the fire is not quenched” (Mark 9:48).
Mortalism and Annihilationism
Those who hold to the doctrines of Mortalism and Annihilationism believe that the unsaved will be ultimately destroyed and cease to exist, rather than suffer unending torment in hell. Although this belief is prominent among a few Protestants, there is no scriptural support for this belief. Some argue that one receives eternal life and the other eternal destruction, but destruction does not mean something is annihilated, or that it ceases to exist. Years ago, I accidentally destroyed my mower by running over a tree stump. It bent the shaft. It was ruined. It was destroyed, but it didn’t cease to exist. It was sent to a recycle center, but that steel is still around somewhere and in some form. It didn’t evaporate into nothingness, so just because the Bible sometimes speaks of hell as eternal death or destruction, does not mean we can insinuate this means they go out of existence. This is not what Jesus taught (Mark 9). Some theologians argue that conditionalism and annihilationism go together, but scripturally, they do not because these ideas are not totally equivalent.
Conditional Immortality is the belief that not everyone that has ever lived will be raised to immortality. They feel that only the saved will live forever, however we can find many Scriptures supporting the belief that every will live beyond the grave in one place or another. There is no “second chance” taught in the Bible, as some cults teach. Conditionalists do not believe those punished in hell will live forever, believing that they will burn up and cease to exist, and it is an irreversible death from which there is no escape, return, or repentance possible. They believe that even their conscience ceases to exist, so why would Jesus warn those about hell if it was just a few terrible minutes and it’s all over? Why would Jesus warn that “if your hand causes you to sin, cut it off? It is better for you to enter life crippled than with two hands to go to hell, to the unquenchable fire” (Mark 9:43), but if the hands cease to exist, what’s the big deal about having two of them in hell? They and their hands are going to cease to exist anyway, right? No, we know this is false doctrine. This would mean the most heinous villains in history (like Hitler) and that nice lady across the street who died without Christ would meet the same fate, and to me, that is not justice. We know that God will judge justly. You can count on that!
When we try to water down the doctrine of hell, we remove some of the natural fear of God by taking out certain biblical teachings such as judgment, wrath, grace, and mercy. Fear is healthy. It keeps us from hurting ourselves and others, and a reverential, holy fear of God shows we take Him and His Word seriously. We don’t try to add to or take away from the gospel. The gospel will offend. It cannot be helped. Remember the Apostle Paul’s words about the gospel when we share it with others. We are “to one a fragrance from death to death, to the other a fragrance from life to life. Who is sufficient for these things” (2 Cor 2:16)? We are not sufficient in ourselves. We need the Spirit of God and the Word of God, given by a person of God to birth the children of God. But this is all for the glory of God.
Here is some related reading for you: Will Believers have a Physical or Spirit Body in Heaven? 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), Crossway Bibles. (2007). ESV: Study Bible: English standard version. Wheaton, Ill: Crossway Bibles. Used by permission. All rights reserved.