Answer:
Good question and a big one.
This is a powerful passage that must be studied in context to fully understand it. Isaiah is talking about the salvation of all the nations through Israel's God – but I don't think his intent is to teach that every human will be saved in eternity someday. And we must not interpret this passage in violation of all others and there are many others that teach many will remain rebels to the end. Jesus himself made this abundantly clear. (Matt 7:14; Matt 25:41).
So when we look at the whole passage, we see that Isaiah is talking about how all the nations will one day come to God and put aside their idols and receive life from Him (Isa 45:20-22). We tend to think of this scene of everyone bowing happening in heaven someday, but I suspect that Isaiah has in mind a temporal view of the final “Day of the Lord”. He talks over and over about the consummation of history, when Israel is redeemed, back in the land, and all nations serve the God of Israel (Isa 2:2,3).
Isaiah is mentioning that all peoples will find salvation through Israel’s God – and so this foreshadows how the message of salvation through Christ would be to “all nations”. However, his emphasis is not the extent of the salvation (IE, everybody is saved) but rather the source of the salvation (Israel’s God vs. other gods).
Jews simply believed that their God was everyone's God – maker of heaven and earth. So "every knee" emphasizes that NOT JUST ISRAEL'S knee will bow someday. Not just Israel's tongue will confess, but other nations too will see the truth. It’s a statement of how international God's concern is, and a prediction of Jesus mission. Therefore, I don’t think this is even implying that every human being will be saved. It’s saying, salvation comes ONLY through Israel’s God (vs. 21) and that he is available to be a savior to more than just Israel (vs. 22).
So another question then is, how can this predicted confession of Israel’s God by every tongue not be a “saving confession”? Well Isaiah says, some of the confessors will be those mad with God and will come to him and be ashamed (Isa 45:24). So this confession may bear no real relationship to the requirements for receiving God's grace. It’s just a confession that their gods were no gods at all.
Keep this in mind: when Paul quotes Isaiah he does so envisioning, not a throng of people confessing sins and fully submitting to Christ, but a Judgment Day, when we all give an account.: Rom 14:11-12
"'As surely as I live,' says the Lord,
'every knee will bow before me;
every tongue will confess to God.'"
…So then, each of us will give an account of himself to God.
Paul doesn't see this vision in Isaiah as a point of potential salvation after death for all, but rather the point in eternity when who God is, is made plain to ALL and all will acknowledge it, whether they want to, OR NOT. They bow volitionally or some, NOT. He repeats his view when quoting Isaiah again in Phil 2:11, equating Messiah with Israel’s God and the confession there could again be willing or non-willing.
We might wonder if this confession could be a "SAVING" confession because the NIV says they confess "allegiance". But that word is not in the original Hebrew. The language is simply, “knee bow, tongue swear before me.” That's it. So in real terms, this could be simply the begrudging acknowledgment, like when a convict "confesses" before a judge. Truth has been acknowledged, but the heart hasn't changed.
What the Isaiah passage affirms is that the salvation through Israel's God is not based on their own moral muscle or goodness, but rather Gods: "the LORD is the source of their strength and righteousness." (Isa 45:24) So once again, no matter who gets saved or when they get saved, the Bible is consistent: they are saved by God's grace. "All the generations of Israel" (vs. 25) will be justified by this grace… a reference to salvation coming to all sons of Abraham – and we know from Paul that spiritual Israel includes all who walk by faith in God's Messiah (Rom 4:16).
This passage does make us wonder, is there another chance, after death for those who reject God’s grace in this life? Why wouldn’t God give another chance after death?
But what we can assume, based on God's heart for the lost (2 Peter 3:9) is that if a million chances would do any good, God would give them. I say to people, don't worry, the "judge of all the earth will do right" in regards to the unrighteous. (Gen 18:25) But we are lead to believe that there will be no second chances (Heb 9:27). The problem is not God’s stubbornness, it's that WE harden in our decisions and solidify along the lines of the moral choices we make in this life. Our lives are, as one Christian said, “the veil of soul making”. Meaning, life is our chance to make our souls as they will always be, in the same way a clay pot has a short formation phase that gives way to a finished product – what it will be forever. Gradually it’s malleability weakens as the clay hardens. The chance for chances – for change – is eventually done. CS Lewis says, “it doesn’t take a lot of faith to believe that God knows when that time comes."
But what is life, if not a million second chances? But we think they're not REAL chances. We think that in eternity, when someone finally SEES God they will of course see their error and bow in a true and contrite confession of trust and allegiance. We assume that that will the first REAL chance they had to turn. Nonsense.
Paul uses the Isaiah passage to teach that ALL may in fact bow and confess God’s lordship, but that doesn't mean they will all turn. Seem impossible? But what about the drunkard now? He sees his error, he may even confess his sin when pressed, and yet he goes right back to the bottle he loves MORE than his chance at freedom. His best chances at freedom were when he was a child before he ever picked up a bottle, even before he understood what the consequences of drinking would be, before he became hardened in this condition.
Like that, our best "chance" to choose God's redemption might be on earth without God visibly before us, rather than in eternity, in judgment, bowing and confessing unwillingly. That might only harden us more.
