"You claim that there is NO SUCH ASSURANCE (do you not?)"
If we abide in the Vine, then certainly. If not, then not.
There really is nothing to be said about a warning from God not to allow something to happen with us, if it cannot possibly happen with us. If it is impossible to fall from the Vine, then the warning from God is vain and useless and confusing, which of course God's Word is none of these.
Likewise if it were impossible to fall from grace, then Paul has lied and said ye are fallen from grace.
The covenant of the law and the covenant of grace do not differ in requirement to behave ourselves as participants therein. The covenant of the law was to teach us that very thing: that God does command and demand outward obedience to His will. That covenant was weak in that it only commanded outward obedience, but that commandment remains in the new testament that first commands inward faith and circumcision of the heart.
God still will not justify the wicked (Ex 23:7), whether they believed in Egypt and then rebelled in Sinai, or they believed in sin, and then rebelled in grace.
He has now changed the means of obedience, not the necessity of obedience.
We now have a more sure assurance of fulfilling His righteousness, but that two-edged sword also means we now have less excuse for not doing so, because we have greater promises of the Spirit within, than the Lord on the mount. They had the blood of the lamb upon their doorposts, and the law from the mount. We have the blood of the Lamb upon our souls, and the law from heaven written upon our hearts.
Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace? (Heb 10)
There really is nothing to offer here that could unsay the Scripture: The heathen do not betray God, having never been known by Him and trusted with the blood of His Son. Only friends can betray friends.
They who were sanctified now do despite to the Spirit of grace. He that was saved, and is not saved... (Rev 17:7)
Look, when it comes right down to it, whether eternal security or not, no one in open transgression and rebellion against God shall inherit the kingdom of God. So, if it makes you feel better to accuse such as hypocrites from the start, who only professed Christ with the lips. Then fine. And if it also gives you assurance when you sin or transgress, that you are still saved. Then fine too. So long as you confess, you shall be forgiven. (But then if future sins are already forgiven, no need to confess?)
If I sin and transgress, and God shows me as much by His Word, then I take Him at His word and confess unto forgiveness and repentance, knowing that by grace and the shedding of His blood I even have opportunity to do so.
I will say one thing about absolute perfect assurance from God: If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness. (I John 1)
We don't have to beg, nor plead, nor ask a hundred times. The very moment God sees our agreement with Him in our hearts, that we have sinned, then He is faithful and just to forgive us straightway, completely. And it is usually we who have the problem of forgiving ourselves and others, even as He forgives us (Matthew 18:35). And frankly, I don't think he wants to here any promises of repentance. Just be forgiven and thankful and free and clean and go and do it no more. And if we do, He is faithful and just to forgive us by our confession 70 times 7.
However, Scripture speaks plainly of a point where repentance is impossible to renew, because confession is impossible to make, because God has given up on us in our openly unashamed shaming of His Son, without any desire to do good. (Heb 6:6-9) (Titus 1:16)
Yes, Scripture shows that god can indeed cease to draw us to Himself, and without such, we cannot possibly come to Him...anymore:
No man can come to me, except the Father which hath sent me draw him: and I will raise him up at the last day. (John 6:44)