To begin with we know that it is possible to gain Salvation and that Salvation is necessary. A clear example of the possibility of Salvation and the possibility of something not else if we do not gain Salvation is shown in John 5:29 and come forth, those who have done good, to the resurrection of life, and those who have done evil, to the resurrection of judgment.
Now that we have established there is Salvation, but that there is something other than Salvation (please refer to Is Salvation Necessary for further details on the necessity of Salvation) we are called to grapple with the question of is Salvation granted for all times, or can it be lost. Many Christians do not believe that Salvation can be lost and cite John 5:24 Truly, truly, I say to you, he who hears my word and believes him who sent me, has eternal life; he does not come into judgment, but has passed from death to life or other similar Scripture passages as proof that Salvation is forever.
Since the Bible is the Word of God and therefore correct on the basis of John 5:24 we do receive eternal life. Since the Bible is the Word of God it cannot contradict itself. It is in this inability of the Bible to contradict itself that the possibility of losing Salvation becomes evident. While Christ was on Earth He spoke of an unforgivable sin as shown in Matthew 12:31Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven. What this means in plain English is Knowing that He was to die to forgive the sins of man Christ still said some sins are unforgivable! This means if you were to say “The Holy Spirit (insert blasphemous statement)” you would have committed an action that is not forgivable.
This does not show that you can lose Salvation though. It only shows that not all sins are forgivable. If Salvation can be lost it must have Scriptural evidence to that effect after the Resurrection. As shown in Jude 1:4 I say this because some godless teachers have wormed their way in among you, saying that after we have become Christians we can do just as we like without fear of God's punishment. The fate of such prople was written long ago, for they have truned against our only Master and Lord, Jesus Christ we can not act as we want to without fear of punshment. As shown in Hebrews 10:26-31 For if we sin deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a fearful prospect of judgment, and a fury of fire which will consume the adversaries. A man who has violated the law of Moses dies without mercy at the testimony of two or three witnesses. How much worse punishment do you think will be deserved by the man who has spurned the Son of God, and profaned the blood of the covenant by which he was sanctified, and outraged the Spirit of grace? For we know him who said, "Vengeance is mine, I will repay." And again, "The Lord will judge his people." It is a fearful thing to fall into the hands of the living God punishment is a possibility.
Now that we know through Scripture that after the Resurrection punishment is still possible it must be shown that the punishment is in fact the loss of Salvation. Since the end result of Salvation is inheriting the Kingdom of God, the punishment of losing Salvation is not inheriting the Kingdom of God. This is shown in Galatians 5:19-21 Now the works of the flesh are plain: fornication, impurity, licentiousness, idolatry, sorcery, enmity, strife, jealousy, anger, selfishness, dissension, party spirit, envy, drunkenness, carousing, and the like. I warn you, as I warned you before, that those who do such things shall not inherit the kingdom of God
Scripture also shows that our eternal life can be removed. This is shown in Rev 22:18-19 For I testify to every one that heareth the words of the prophecy of this book: If any man shall add to these things, God shall add unto him the plagues written in this book. And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from these things that are written in this book
While it is a very disquieting thought Scripture does show that Salvation can be lost. Christ says that not all sins are forgivable (Matthew 12:31), we can be punished even if we accept that Christ is the Savior who died for our sins (Hebrews 10:26-31), the punishment can be the loss of the inheritance of the Kingdom of God (Galatians 5:19-21), and eternal life can be removed (Revelation 22:18-19)