I am personally of the belief that all those who fall away were never regenerated in the first place, or born again of the spirit of God. This passage got my attention a while back.
John 6:60 Many therefore of his disciples, when they had heard [this], said, This is an hard saying; who can hear it?
John 6:61 When Jesus knew in himself that his disciples murmured at it, he said unto them, Doth this offend you?
John 6:62 [What] and if ye shall see the Son of man ascend up where he was before?
John 6:63 It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, [they] are spirit, and [they] are life.
John 6:64 But there are some of you that believe not. For Jesus knew from the beginning who they were that believed not, and who should betray him.
John 6:65 And he said, Therefore said I unto you, that no man can come unto me, except it were given unto him of my Father.
John 6:66 From that [time] many of his disciples went back, and walked no more with him.
John 6:67 Then said Jesus unto the twelve, Will ye also go away?
John 6:68 Then Simon Peter answered him, Lord, to whom shall we go? thou hast the words of eternal life.
John 6:69 And we believe and are sure that thou art that Christ, the Son of the living God.
John 6:70 Jesus answered them, Have not I chosen you twelve, and one of you is a devil?
John 6:71 He spake of Judas Iscariot [the son] of Simon: for he it was that should betray him, being one of the twelve.
I used to go back and forth with OSAS, but ultimately came to the conclusion that if one is truly born again and become a new creature in Christ and been adopted as a son then it would be profoundly hard to fall away, which is why I believe that those who fell away were never regenerate. The Bible says we are sealed until the day of redemption and that no man can pluck us out of the fathers hands. Even in the book of 1st Corinthians there was a man among them that slept with his fathers wife and was chastised with the destruction of the flesh so that his spirit may live.
There are times I have backslid, but think about what it would take for someone who is a son of God and born by the spirit of God to fall away. If you have ever backslidden as a Christian you know it is a miserable place to be, because the whole time you are grieving yourself by grieving the spirit of God. To remain in a constant place of grieving the spirit of God is to remain in a place of great sadness and torment to the Christian. Not only that, but Gods word promises to chastise us in the book of Hebrews. He chastises us as his son.
Heb 12:5 And ye have forgotten the exhortation which speaketh unto you as unto children, My son, despise not thou the chastening of the Lord, nor faint when thou art rebuked of him:
Heb 12:6 For whom the Lord loveth he chasteneth, and scourgeth every son whom he receiveth.
Heb 12:7 If ye endure chastening, God dealeth with you as with sons; for what son is he whom the father chasteneth not?
In order for a born again Christian to completely fall away he would have to endure and than somehow bypass not only his own nature as a new creation that shares the same disposition of sin as God, but also bypass Gods rod of correction as a loving father. I have backslid allot, but I have never been able to fall away to the point where I was able to bypass living in godly sorrow and a constant state of grieving the holy spirit, and then on top of it bypassing Gods rod of correction. He goes out and searches for that one sheep that is lost while leaving the 99 behind. If you truly belong to Christ than God will find you. Recently I was in a place where I could no longer feel conviction over sin, I couldnt feel God, nothing, but then one day in a near death experience God had visited me and brought me to repentance. Sometimes thats what it takes and sometimes thats what God allows.