Obviously I don't know what Scheffler's correspondent meant in a theological sense. But the grace made accessible through Christ's death on the Cross is (a) sufficient for everyone and (b) always available. And Christ's victory over death is complete and irreversible, although He has not yet (to continue the military metaphor) fully exploited that victory by putting an end to the natural order that resulted from the fall of man.
So sure, one's victory is secure on the cross.
The question that matters to the individual, though, is whether one is going to take advantage of the victory Christ won.
Is it presumptuous to think one has irreversibly done that? Yes, dangerously so.
Contemporary American evangelical Christians frequently posit that sanctifying grace is (a) irresistible and (b) obtained solely through the act of faith, such that virtuous works are merely evidence of the state of one's soul (not a cause of such state) and that no discrete act of repentance is ever necessary for sin. The Protestant quandary was classically "do I really have faith?" (if you do you'll be saved regardless of what you do, but if you don't you won't be saved, also regardless of what you do), but contemporary American Christians of all stripes tend to assume that anyone they liked, particularly anyone they liked who professed to be Christian at death, has been saved. Much less Scarlet Letter, much more All Dogs Go to Heaven.
The Church has always taught that one cannot have perfect knowledge of the state of one's soul, but that the properly formed conscience can get a pretty good idea, and that the sacraments are efficacious for those who receive them properly (even if we can't ever have perfect knowledge of who has). The textbook answer to "are you in a state of grace?" is the one St. Joan of Arc gave, "if I am, I pray that I may remain in it, and if I am not, I pray that I may be restored to it." Or, as Chesterton put it, "every man is damnable, but no man is damned."