It's been ages since I've been on - and I'm afraid I haven't read all 46 pages of the thread, so if this is a repeat offense, please forgive the repetition.
Ultimately, I think the answer is no - but there are probably a lot fewer people who are/will be saved than claim it.
Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God.
I John 3:9
It is axiomatic that "you must be born again" to enter the kingdom of heaven. Anyone who is born of God - by definition - is saved and is kept from falling. However, there are some passages that make one wonder....
For there are certain men crept in unawares, who were before of old ordained to this condemnation, ungodly men, turning the grace of our God into lasciviousness, and denying the only Lord God, and our Lord Jesus Christ.
I will therefore put you in remembrance, though ye once knew this, how that the Lord, having saved the people out of the land of Egypt, afterward destroyed them that believed not.
And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.
Even as Sodom and Gomorrha, and the cities about them in like manner, giving themselves over to fornication, and going after strange flesh, are set forth for an example, suffering the vengeance of eternal fire.
Likewise also these filthy dreamers defile the flesh, despise dominion, and speak evil of dignities.
Yet Michael the archangel, when contending with the devil he disputed about the body of Moses, durst not bring against him a railing accusation, but said, The Lord rebuke thee.
But these speak evil of those things which they know not: but what they know naturally, as brute beasts, in those things they corrupt themselves.
Woe unto them! for they have gone in the way of Cain, and ran greedily after the error of Balaam for reward, and perished in the gainsaying of Core.
These are spots in your feasts of charity, when they feast with you, feeding themselves without fear: clouds they are without water, carried about of winds; trees whose fruit withereth, without fruit, twice dead, plucked up by the roots;
Jude 1:4-12
The description is clearly one of unrighteousness - but the context Jude gives it is first of all of Israel (people) having been saved from Egypt and THEN walking in unbelief and being destroyed. Next, he refers to angels who "kept not their first estate, but left their own habitation" with predictable results. The letter then makes a direct comparison as Jude says "likewise" - speaking about those who turn the grace of God into lasciviousness and deny Christ. And while the text certainly makes it clear that these people in no way resemble born again believers, the idea that they are "twice dead" only serves to bolster the implication that - like those who were saved out of Egypt and angels that belonged in heaven - these men were once "saved". At least in some sense of the word.
Revelation also has some difficult words. Here is the standard for those who will enter the Kingdom:
And there shall in no wise enter into it any thing that defileth, neither whatsoever worketh abomination, or maketh a lie: but they which are written in the Lamb's book of life.
Revelation 21:27
So the bottom line is that one must be written in the Lamb's book of Life. This is no surprise since we've already read this:
And whosoever was not found written in the book of life was cast into the lake of fire.
Revelation 20:15
Revelation 13:8 echoes that same sentiment. and we even read this:
The beast that thou sawest was, and is not; and shall ascend out of the bottomless pit, and go into perdition: and they that dwell on the earth shall wonder, whose names were not written in the book of life from the foundation of the world, when they behold the beast that was, and is not, and yet is.
Revelation 17:8
But is that something set in stone? "From the foundation of the world" sounds pretty solid - however, could that simply imply that the name was never erased?
He that overcometh, the same shall be clothed in white raiment; and I will not blot out his name out of the book of life, but I will confess his name before my Father, and before his angels.
Revelation 3:5
This is phrased in the negative ("....will not blot out...") so is it possible that it is just a figure of speech to note that one is written in the book? Or can it be blotted out?
For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are 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 the things which are written in this book.
Revelation 22:18-19
There it is in the positive sense as well ("...shall take away...").
Of course, there are also all those passages from Hebrews that are commonly used - and even the idea of "the sin unto death" from I John (the book I started this post with).
If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it.
All unrighteousness is sin: and there is a sin not unto death.
We know that whosoever is born of God sinneth not; but he that is begotten of God keepeth himself, and that wicked one toucheth him not.
I John 5:16-18
There again we see John saying what he said in chapter 3. And I suspect this passage, too, has been referred to in the thread. We can discuss if the sin unto death is physical or spiritual death - and maybe that's even a thread on its own. But what is the point of asking for life for someone who has not sinned unto death - if that is a physical death? If it isn't unto death, then asking for life...what sense does that make (for a physical death)?
We can know we have eternal life :
These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.
I John 5:13
But can it be lost? the scriptures in Jude and Revelation seem to hint strongly that it can. However, those that trust in Him will never be lost.