Called the “Once-Saved-Always-Saved” (OSAS) doctrine, this has historically been the most debated subject among Christian groups. When John Calvin came up with the idea of Divine Predestination—God chooses people to be saved and lost in spite of free will—he had to obviously conclude that salvation could not be lost, ever.
There are many verses that contradict the OSAS belief. Yet a library full of books won't convince the adherents that they are wrong. One such verse is found in Hebrews 10:
If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, but only a fearful expectation of judgment and of raging fire that will consume the enemies of God.
The writer is talking about saved individuals here, not unsaved sinners. Preachers will tell you there is nothing in the verse that says the person was saved in the first place. “After receiving the knowledge of the truth” doesn’t mean you hear a sermon, leave the church building, and then be forever lost because you didn’t respond to the Good News. Note as the section continues:
Anyone who rejected the law of Moses died without mercy on the testimony of two or three witnesses. How much more severely do you think a man deserves to be punished who has trampled the Son of God under foot, who has treated as an unholy thing the blood of the covenant that sanctified him, and who has insulted the Spirit of grace? For we know him who said, “It is mine to avenge; I will repay,” and again, “The Lord will judge his people.” It is a dreadful thing to fall into the hands of the living God. NIV (Heb 10:26-31)
This is not talking about someone blowing off the invite of some street preacher, but of someone who actively went against God's gift. God didn't sacrifice His Son so that a person could thumb his nose at Him so they could go back to a life of sin.
Many denominational preachers are so enamored with Calvin's writings, that they cannot abandon them. Here are a few other verses that contradict the idea of never losing one's salvation:
You might say, “Branches were broken off so that I could be joined to their tree.” That is true. But those branches were broken off because they did not believe. And you continue to be part of the tree only because you believe. Don't be proud, but be afraid. If God did not let the natural branches of that tree stay, he will not let you stay if you stop believing. So you see that God is kind, but he can also be very strict. He punishes those who stop following him. But he is kind to you, if you continue trusting in his kindness. If you don't continue depending on him, you will be cut off from the tree. (Romans 11:19-22)
But now (God) has made you his friends again. He did this by the death Christ suffered while he was in his body. He did it so that he could present you to himself as people who are holy, blameless, and without anything that would make you guilty before him. And that is what will happen if you continue to believe in the Good News you heard. You must remain strong and sure in your faith. You must not let anything cause you to give up the hope that became yours when you heard the Good News. (Col 1:22-23)
Brothers and sisters, I want you to know what happened to our ancestors...God was not pleased with most of those people, so they were killed in the desert. And these things that happened are examples for us. These examples should stop us from wanting evil things like those people did. (1 Cor 10:1-6)
The things that happened to those people are examples. They were written to be warnings for us. We live in the time that all those past histories were pointing to. So anyone who thinks they are standing strong should be careful that they don't fall. (1 Cor 10:11-12)
For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt. For land that has drunk the rain that often falls on it, and produces a crop useful to those for whose sake it is cultivated, receives a blessing from God. But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned. (Hebrews 6:4-8)