Yes, His Word last forever.
Ex. 20:6 And shewing mercy unto thousands of them that love me, and keep my commandments.
His counsel shall stand.
Duet. 30:19 I call heaven and earth to record this day against you, that I have set before you life and death, blessing and cursing: therefore choose life, that both thou and thy seed may live:
Ez. 18:21 But if the wicked will turn from all his sins that he hath committed, and keep all my statutes, and do that which is lawful and right, he shall surely live, he shall not die.
22 All his transgressions that he hath committed, they shall not be mentioned unto him: in his righteousness that he hath done he shall live.
23 Have I any pleasure at all that the wicked should die? saith the Lord GOD: and not that he should return from his ways, and live?
32 For I have no pleasure in the death of him that dieth, saith the Lord GOD: wherefore turn yourselves, and live ye.
Therefore, "today if you hear His Voice, harden not your heart".
Because HIS Counsel will stand.
What God had from the beginning wants for man? That man believes in Him (all the time). So, what God had from the beginning have man choose freely, is to believe God (all the time) or not. God gave a very simple commandment to man, then in the person of Adam, to not eat of the forbidden fruit. And that commandment was not good only for a day, but that it is upon Adam, to believe in God and do as God commanded him, all the time, every day. Adam, one day, had fallen, and continueth not to be faithful to God.
Now, those who come after Adam, have you not learned what they freely choose between believing God (all the time) or not. That's right, it's the latter. No one ever had chosen to be faithful to God. One way or another, they fell and failed to be faithful. If you deny that truth, it's yours to make. But that does not change the fact that, that is the reality and truth of the matter.
Do you think for a second that there is even one man who would have not sinned against God and continued to be faithful to Him (except Jesus Christ), and need not to be saved from hell? The point is, if God lets the man be as he is, with his freedom to choose, he surely is going to the lake of fire, for he surely would, like the father of mankind, Adam, be unfaithful, and be guilty of sin. And that is what God, in his foreknowledge, would have seen ahead. It had been proven, that no man could be right with God on and by his own. Only Jesus Christ can and is. In such case, that man will surely not make it, if left to himself, what do you think is left to be done, so that, the kind of creature that man was originally created by God in the beginning be saved and restored to what it formerly was?
This is what we know in scriptures. God had by election of grace, chosen a people, even from before the foundation of the world, whom He will create anew, conformed to the image of His Son, Jesus Christ. Who are these people? They are those who are being saved, those who are in Jesus Christ.