So what's with the disbelief of passages such as John 10:28?
John 10:27-29 "My sheep hear my voice, and I know them, and they follow me: And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand. My Father, which gave them me, is greater than all; and no man is able to pluck them out of my Father's hand."
This is a determined statement by Christ saying that nobody will perish whom he saves, because they have eternal (unending) life; and that no man (which would include the person being saved, because that person is a man/woman) can pluck them out of his hand. It's not like we can run away from God once we're saved, we can just see Jonah and how that worked out for him. Though in Jonah's case, it was a specific work, the love of God towards the saved is exactly the same.
The problem that we run into with saying a person can lose their salvation by falling into sin is this: where do we draw the line? Is the line specifically stated in scripture that once a Christian passes this certain point in sinning that he is no longer saved? Because I've never found such a line drawn in scripture. The one thing we know for certain is that there is none righteous (Romans 3:10, Romans 3:23), and that we all deserve death as a consequence of our sin, both physically and spiritually (Romans 5:12, Hebrews 9:27). So, sin being the problem, and Christ being the perfect solution to take away the problem, how is it that adding more of the problem can take away the perfect solution that is Christ? We know that Jesus is perfectly sufficient for the sins of the world (1 John 2:2, 1 Timothy 4:10, John 3:16), so is it fine to simply say that sin can become bigger and outweigh the sacrifice of Christ? No, it's not.
Again, John 10:28 perfectly refutes the idea of "forfeiting" one's salvation. Why would God take back something he already gave? It's a gift. Isn't a gift something that is given permanently to someone else bought and paid for by the giver? The buyer won't take it back, because it's not a loan, nor did the receiver borrow it. The receiver now possesses the gift. And if salvation belongs to the Lord, the Lord is the only one who can receive back that gift, except he won't.
Hebrews 6:4-6 "For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame."
So ripping this verse out of context doesn't do much good to the argument of losing one's salvation. This verse would contradict many other scriptures saying a person can lose their everlasting life (which is an oxymoron by the way), if that were the case. The verse is stating that it is not possible for those people who received the Holy Ghost (were saved) that if they should fall away, to renew them again unto repentance. Why? Because they are already renewed. They would put Christ to an open shame if they were re-renewed, because that would show that Christ Jesus wasn't the once-for-all perfect sacrifice for sins, and it would disgrace his name as unworthy. Except that's not possible, because Christ is the perfect sacrifice for sins for the entire world.
Remember the verse in Ephesians 4:30, that says we are sealed by the Spirit UNTO the day of redemption pictured in 1 Thessalonians 4:15-18? We are sealed with that Spirit once we believe and trust the gospel of Christ (Ephesians 1:13). That would contradict the passage in Hebrews 6 if a person could lose their salvation: they would lose their seal, even though all who are sealed are sealed unto that day according to Ephesians 4:30.
And also, God would be a liar if he promised us everlasting life through his Son that had nothing to do with ourselves (Ephesians 2:8), and was a free gift (Romans 5:18); except God cannot lie, as some of you presume (Titus 1:2). The promise of eternal life is given to all who become born of God through faith in Christ, which is not conditioned based upon their personal holiness in any way; it's only condition based upon their belief (John 3:18, Mark 16:16).
The fact is, if we had to live our lives a certain way or stop sinning in order to maintain salvation, none of us would be saved, and salvation wouldn't be by grace. We all sin everyday (Proverbs 20:9, Ecclesiastes 7:20, John 1:8, John 1:10), so that would automatically disqualify our salvation each time we awoke if what some believed in this thread were true, and that would put Christ to an open shame.