So also as regards the gift of perseverance, of which it is written, He that shall persevere to the end, he shall be saved:-which gift cannot be derived from any other but Him, who is able to establish him who standeth that he stand perseveringly, and to restore him who falleth:-let no one herein promise himself any thing as certain with an absolute certainty; though all ought to place and repose a most firm hope in God's help. For God, unless men be themselves wanting to His grace, as he has begun the good work, so will he perfect it, working (in them) to will and to accomplish. Nevertheless, let those who think themselves to stand, take heed lest they fall, and, with fear and trembling work out their salvation, in labours, in watchings, in almsdeeds, in prayers and oblations, in fastings and chastity: for, knowing that they are born again unto a hope of glory, but not as yet unto glory, they ought to fear for the combat which yet remains with the flesh, with the world, with the devil, wherein they cannot be victorious, unless they be with God's grace, obedient to the Apostle, who says; We are debtors, not to the flesh, to live according to the flesh; for if you live according to the flesh, you shall die; but if by the spirit you mortify the deeds of the flesh, you shall live.
It's not, as they say, that God is an uncertain rock on which we may stand. Far from it! Rather, it is we who are uncertain, and they demonstrate this fact from Scripture.
"Wherefore, he that thinketh himself to stand, let him take heed lest he fall" (1Co 10:12). Does this make a lick of sense unless the Christian can indeed fall? To what purpose the warning if he cannot fall?
"Wherefore, my dearly beloved, (as you have always obeyed, not as in my presence only but much more now in my absence) with fear and trembling work out your salvation" (Php 2:12). Does this make a lick of sense if Christians need not fear in some way concerning his salvation nor "work out" their salvation? To what purpose the admonition if they need not do so? This isn't to say that we have to live in terror, obviously, but it likewise doesn't mean that we can blithely go our merry way.
"But in all things let us exhibit ourselves as the ministers of God, in much patience, in tribulation, in necessities, in distresses, in stripes, in prisons, in seditions, in labours, in watchings, in fastings, in chastity, in knowledge, in longsuffering, in sweetness, in the Holy Ghost, in charity unfeigned,…" (2Co 6:4-6). Why does St. Paul exhort them to perseverance in such things if their perseverance is assured?
"For if you live according to the flesh, you shall die: but if by the Spirit you mortify the deeds of the flesh, you shall live" (Rom 8:13). Why does St. Paul warn the Roman Christians against a life according to the flesh if their perseverance is assured?
To the contrary: if we fall, it is our own fault; God gives his grace for perseverance to those who seek it. That perseverance may not be easy; we may have to live it through tribulations and distresses and stripes and prisons. But God's grace to help us will be there, if we ask him.