Respectfully Bob:
The "staying" power is not in the "clay" or "man", but in God.
To "not endure" and connect it to the "saved" is to negate the omnipotence of God, and also His complete ability to secure our salvation.
Also to lose one's salvation due to neglect, lack of endurance is to change the parameters of what salvation is.
Salvation is a 1. The Old Adamic Nature(identity) is put to death by the power of God, as we are co-crucified, co-buried, co-raised up, and finally co-seated with Christ in the heavenlies. 2. Christ's crucified and glorified life enters our soul at the time of salvation, and displaces the Adamic nature with His sinless nature. 3. "But We Still Sin!!!" Easily answered.....We contend not with the Old Adamic nature that has only one proclivity, and that's to sin and work against God, but we do have our unredeemable flesh or "Earth Suit" still covering us until the we get those glorified bodies. 4. With our redeemed souls still resides the "programming/memories/habits/proclivities" that were part and parcel of our pre-salvation life when we were part of the "lost". Now that we have lost the Old Nature, we still must contend with the old programming of years and years of unGodly life/habits/ways, with the commands of Romans 12 that start right out with the exhortation to "renew" one's mind with the "washing" of the Word. The Word displaces the "lies" that we previously lived by.
Here is the "corker". When we receive salvation/Christ's life, we receive His life that endured crucifixion, burial, ressurrection, and heavenly ascension +or - 2,000 years ago. I.E. within our saved souls we have partaken and joined with Christ in His life that has no beginning, nor an end..........It is eternally infinite both in the past/present, and future. It also indured Calvary, enjoyed ressurrection, and was glorified in ascension.
Coursing in our soul is the a life that in not earth bound, but is seated in the heavenlies in Christ Jesus.
The Flesh: This is our foe along with Satan and his principalities, and also the "ways" of the Old Adamic life. This is why we "soldier on" in the strength of Christ.
We are not expected to endure without the empowerment of God. When it "appears" that we have failed to do so, then we must look at Pauls statement where this "thorn" in the flesh just kept him frustrated as he emplored God to remove it. God did not remove that impediment to Pauls walk as a Christian. Paul finally got the "light". God was convincing he/Paul that this thorn was for a reason. It kept Paul in a state of "self/weakness", and forced in to his knees in proclamation "When I am weak He is strong!".
As I mentioned in earlier posts, be very very careful in attempting to discern another's spiritual condition in regards to their being saved or not, or that they've somehow overpowered God's sheepfold and snuck away into lost-abandon and hell.
Also, mentioned in the Pauline epistles is the very clear statement that when believers continue to sin, and make public embarrassment of the work of salvation in their lives by their God, that at God's discretion He may take them home. Yes, their works may in most part be wood, hay and stubble, but never the less they are still His.
Because of this human freewill that is not hindered but is a means of magnifying God's grace in the human race before the myriads of unsaved of the world, it goes only so far.
Do not let every ill-appearance/act of a person who claims to be saved to be an indication of one walking away from the sheepfold. Remember again, that salvation is not merely an act of choosing, and holding-onto the One who does the saving, but is a bonifide "transformation" of the soul from top to bottom. Yet, we still have our original minds, along with our original sources of emotions, and of course our human wills. Within that those elements lies the throne-room of the human soul. There is but one throne at the center of the mind/will and emotions, and it either has Christ sitting upon it or ourselves. When us Christians live in the flesh, we have displaced Christ off of the throne of our soul and placed ourselves. Now we start to go into legalism, dead works, and basically agree with the antagonist of our souls, namely Satan himself. Our unredeemed flesh allows access of the enemy of our souls to our mind, emotions, and even the stifling of our wills to some extent. Paul's weakness, that allowed God to take the helm of His life, is of the same category. We as Christians will never, never walk in obedience to the Spirit that endwells us 100% of the time. What should happen as we endure this world, and the ugly works of the flesh and what they bring upon us, is an ever-lessening of these bouts where we are living fleshly. The "endurance" is the work on our part through cooperation with God's Truth, and the empowerment of the H.S. to "choose" or exercise our "wills" in conjunction with said-truth, and not the old ways still imbedded in our grey matter.
Paul's Roman's 7 struggle would have to be defined by the "lose one's salvation" group as a description of Paul the Apostle falling in and out of salvation, wouldn't it? Afterall, in his Roman's 7 discourse he laments that fact that he does the things that he shouldn't, and doesn't do the things he should..........."What a wretch I am!!!!"
This is the laments of a fully secure, saved, human being! He is having an internal struggle with who will take the helm or throne of his soul. Paul is helping us fellow Christians to define where the problem lies when we do things that make us appear to be "lost" and not "saved".
Remember that the conditon, "flesh" as used properly in the KJV, and NASB, is not origined in the Adamic Nature. That nature is gone.
To say that we can fall out of God's secure sheepfold, is to negate God's staying power to hold us, though as little dumb lambs we will wander, despite our "Shepherd's" exhortations to stay with the flock and listen to His words, and obey.
Those that appear lost and fallen from salvation, are no different that that sheep that gets caught in the briars and brambles of the thorn bush, yet is still one of His.
Adoptions are irrevocable with God. God never revoked His union with Israel, but He did exact a lot of discipline upon it, didn't He?
When He says in Revelations, "I never knew you", that means that there "never" was a relationship from the start. I.E. we have here human beings who appeared to be His, but delusionally, ignorantly, or through willfull deception, were not his.
"Of His" is a key two-word phrase........It means to be part and partial or part of Him. When the scripture says that they weren't, yet followed Him, this distinctly shows the precipice between them and Him, and the relationship of true believers with Him.