The Greek for "uttermost" is παντελὲς (panteles). This is the only place in the New Testament where it is used, except negatively, "in no wise," in Luke 13:11.
It is a strong compound word, meaning "all to the end." The R.V., margin, is "completely." This is its true meaning "perfectly, completely, to the very end" says Delitzsch, "but without necessarily any reference to time." Again he says, "Christ is able to save in every way, in all respects, unto the uttermost; so that every want and need, in all its breadth and depth. is utterly done away." This annotation is a perfect answer to his argument in his Biblical psychology in proof of "the unabolished antinomy' in Rom. 7. "The law in the members" warring until death against the law of the mind, and bringing the Christian at his best earthly estate into captivity to the law or uniform sway of sin. Let us believe the exegete rather than the theologian. It is always safer to trust an honest and scholarly expounder than a warped and traditional dogmatist. Modern interpreters unanimously reject the idea of some of the ancient annotators that "uttermost" has here reference to illimitable future time. Besides being unscholarly, this view involves the heresy of Canon Farrar's "eternal hope" for wicked souls after death.
Why was Paul constrained to invent these new and strong terms? Because he was divinely called to describe what never existed before Pentecost, and for that reason had no name — human souls entirely sanctified through the mission of the Comforter. Why did he not do the same wonderful works before Pentecost, seeing that as God he was omnipresent and omnipotent? He had not the same tools to work with, the completed facts of the gospel ending with the ascension of Christ from the footstool to the throne. "Sanctify them through the truth."
— edited from Half-Hours With St. Paul, Chapter 16.