Many Protestants believe in a once-saved-always-saved doctrine. They believe a true Christian can never lose his salvation. They will point to verses such as Heb. 10:14:
For by one oblation he hath perfected for ever them that are sanctified.
Rom: 8:38-39:
For I am sure that neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come, nor might, Nor height, nor depth, nor any other creature, shall be able to separate us from the love of God, which is in Christ Jesus our Lord.
and John 10:28:
And I give them life everlasting; and they shall not perish for ever, and no man shall pluck them out of my hand.
We can answer the Hebrews quote by saying that the sacrifice of Christ does prefect forever those that are sanctified provided we never sin again. Each time we sin, we turn to Christ and His sacrifice, which once again perfects us forever unless we sin again. A few verses later, we are told that Christians can lose salvation.
We answer the Romans quote by noting that it’s true that no one and no thing can make us cease or even hinder our love for Christ. However, we can choose by our own free will to stop loving Christ. God doesn’t make us love Him.
We answer the St. John quote by noting that God is speaking about His elect of whom only He knows. We don’t know who belongs in the group. I’m sure many damned individuals believed themselves to be part of the elect. Even St. Paul was not sure for he told us in I Cor. 9:27:
But I chastise my body, and bring it into subjection: lest perhaps, when I have preached to others, I myself should become a castaway.
These proof-text Bible verses do not support eternal security for all Christians. The fact remains that true Christians can lose their salvation as the Bible clearly tells us.
He who endures to the end will be saved. (Matt. 24:13, Mark 13:13, James 1:12, Matt. 10:22)
We must endure to the end or else we will not be saved.
John 15: 6:
If any one abide not in me, he shall be cast forth as a branch, and shall wither, and they shall gather him up, and cast him into the fire, and he burneth.
To be cast off as a branch, one must first be a member of the tree. This verse implies that a member of Christ can be cut off to be burned.
Rom 11: 22-23:
See then the goodness and the severity of God: towards them indeed that are fallen, the severity; but towards thee, the goodness of God, if thou abide in goodness, otherwise thou also shalt be cut off. And they also, if they abide not still in unbelief, shall be grafted in: for God is able to graft them in again.
Again, to be cut off implies that one was a member first. Thus, salvation can be lost.
I Cor. 6: 9-11:
Know you not that the unjust shall not possess the kingdom of God? Do not err: neither fornicators, nor idolaters, nor adulterers, Nor the effeminate, nor liers with mankind, nor thieves, nor covetous, nor drunkards, nor railers, nor extortioners, shall possess the kingdom of God. And such some of you were; but you are washed, but you are sanctified, but you are justified in the name of our Lord Jesus Christ, and the Spirit of our God.
St. Paul is warning those who have been sanctified and justified that they could be deceived and go back to their old ways for such people will not be saved. Period!
I Cor. 15: 1-2:
Now I make known unto you, brethren, the gospel which I preached to you, which also you have received, and wherein you stand; By which also you are saved, if you hold fast after what manner I preached unto you, unless you have believed in vain.
St. Paul qualifies his teaching with an “if” or else.
Heb. 3:12-14:
Take heed, brethren, lest perhaps there be in any of you an evil heart of unbelief, to depart from the living God. But exhort one another every day, whilst it is called to day, that none of you be hardened through the deceitfulness of sin. For we are made partakers of Christ: yet so, if we hold the beginning of his substance firm unto the end.
This is another warning to those in Christ that they could possibly lose their salvation.
Heb. 6:4-6:
For it is impossible for those who were once illuminated, have tasted also the heavenly gift, and were made partakers of the Holy Ghost, Have moreover tasted the good word of God, and the powers of the world to come, And are fallen away: to be renewed again to penance, crucifying again to themselves the Son of God, and making him a mockery.
Only a Christian can be made a partaker of the Holy Ghost. He can lose his salvation.
Heb. 10:26-29:
For if we sin wilfully after having the knowledge of the truth, there is now left no sacrifice for sins But a certain dreadful expectation of judgment, and the rage of a fire which shall consume the adversaries. A man making void the law of Moses, dieth without any mercy under two or three witnesses: How much more, do you think he deserveth worse punishments, who hath trodden under foot the Son of God, and hath esteemed the blood of the testament unclean, by which he was sanctified, and hath offered an affront to the Spirit of grace?
II Peter 2:20-21:
For if, flying from the pollutions of the world, through the knowledge of our Lord and Saviour Jesus Christ, they be again entangled in them and overcome: their latter state is become unto them worse than the former. 21 For it had been better for them not to have known the way of justice, than after they have known it, to turn back from that holy commandment which was delivered to them.
These two verses are echoing each other. It’s better to never be a Christian than to stop being Christian.