Since the garden of eden man has been given free will and just as we chose to repent and turn unto the Lord and accept salvation which is found in Him (2 Timothy 3:15), we can also by free will choose to deny Christ and return to the sin we were once saved from! (2 Peter 2:20-22) For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning. For it had been better for them not to have known the way of righteousness, than, after they have known it, to turn from the holy commandment delivered unto them. But it is happened unto them according to the true proverb, The dog is turned to his own vomit again; and the sow that was washed to her wallowing in the mire.
In Christ we are the righteousness of God, but if we turn from our righteousness we will then die in our choice to live in sin again, and forfeit our salvation (Ezekiel 18:24) But when the righteous turneth away from his righteousness, and committeth iniquity, and doeth according to all the abominations that the wicked man doeth, shall he live? All his righteousness that he hath done shall not be mentioned: in his trespass that he hath trespassed, and in his sin that he hath sinned, in them shall he die.
We have just learned our salvation and righteousness are not carved in granite and the doctrine of once saved always saved is conditional upon our choice of the free will we have been given! There is another belief many believe to be permanent yet is not and that is if we are written in the book of life can our name ever be removed? The answer is yes it can as we read when Moses asked to be blotted out of Gods book and was told by God Himself that it is He who would blot out of His book those who have sinned against Him (Exodus 32:30-33) And it came to pass on the morrow, that Moses said unto the people, Ye have sinned a great sin: and now I will go up unto the LORD; peradventure I shall make an atonement for your sin. And Moses returned unto the LORD, and said, Oh, this people have sinned a great sin, and have made them gods of gold. Yet now, if thou wilt forgive their sin; and if not, blot me, I pray thee, out of thy book which thou hast written. And the LORD said unto Moses, Whosoever hath sinned against me, him will I blot out of my book. (Psalm 69:28).
Now the just shall live by faith: but if any man draw back, my soul shall have no pleasure in him. But we are not of them who draw back unto perdition; but of them that believe to the saving of the soul (Hebrews 10:38,39).