I have always had one problem with OSAS, this the free will nature of man and our God not one of compulsion. Though nobody can be plucked from the hand of God, can one renounce God? The only explanation one is going to get is, "Well, that person was never really saved." I honestly don't know that, don't know how anybody else knows that as an absolute, only God able to divide the soul and peer into the heart of any individual man. So, I'll leave this life not knowing if any born again did fall of their own volition, and strongly suspecting this could be so. But that said, the notion that some sin is going to separate the believer from God is wholly error, or that the big sins are more damning than the little ones wholly error. Anybody who says they are without sin, even as a believer, is a deluded liar, who doesn't understand their holy God, or even themselves, in relation to that holy God, how shipwrecked they are, if the blood of Christ doesn't cover all sin, past, present and future. You can fancy yourself Mother Teresa on steroids, but, without the blood of Christ at the time you leave this mortal coil, you are just a filthy sinner, dressed in filthy rags. It behooves some people to consider that the self-righteous were the ones our Lord Jesus was absolutely damning of, the most legendary in their own minds the scum of the earth to His mind. Also, this notion that to accept the security of grace by faith is, as if, some sin license is to ignore the Holy Spirit working in the lives of real believers, as if trying to lay on them the concept of grace meaning works perfection moving forward, now that one is saved a need to put the law corpse on our backs and establish our own righteousness: this is a lie from hell, a lie you can't KNOW you are saved, Satan famous as an accuser and purveyor of guilt and doubt. Tell you what, the "holiest" people are often the most evil, in my experience, nasty, arrogant, self-willed, condemning, just the vain, "born again" Pharisees of scripture in our day, not the broken publican the Lord Jesus saves, the meek and poor in spirit the Lord loves.
Isaiah 6:5 Then said I, Woe is me! for I am undone; because I am a man of unclean lips, and I dwell in the midst of a people of unclean lips: for mine eyes have seen the King, the LORD of hosts.
Luke 18:13-14 And the publican, standing afar off, would not lift up so much as his eyes unto heaven, but smote upon his breast, saying, God be merciful to me a sinner. I tell you, this man went down to his house justified rather than the other: for every one that exalteth himself shall be abased; and he that humbleth himself shall be exalted.
John 13:8-10 Peter saith unto him, Thou shalt never wash my feet. Jesus answered him, If I wash thee not, thou hast no part with me. Simon Peter saith unto him, Lord, not my feet only, but also my hands and my head. Jesus saith to him, He that is washed needeth not save to wash his feet, but is clean every whit: and ye are clean, but not all.
All you Mister Holier-Than-Thous? Tell you what, your feet are still dirty. You'd do well not to kid yourself, or others!