The Hebrews did voluntarily say, "All that the LORD has spoken we will do." The Hebrews, the sons of Israel said that.
Not Israel.
Go back and read what is stated in that chapter. Then do a study of the word "Israel," because you will find "Israel" is not used in the geo-political nation-state definition for several generations, long after those Hebrews, the sons of Israel, clear out and settle the promised land. There is only one single, sole, solitary mention of "nation" in Exodus 19 and that is God's promise to make those who obey Him into a nation of priests. It is a covenant promise, or a promise made inside the God-initiated, already established, Christological covenant of salvation. What does scripture later state about that promise to make a nation of priests?
1 Peter 2:4-12
As you come to him, a living stone rejected by men but in the sight of God chosen and precious, you yourselves like living stones are being built up as a spiritual house, to be a holy priesthood, to offer spiritual sacrifices acceptable to God through Jesus Christ. For it stands in Scripture: “Behold, I am laying in Zion a stone, a cornerstone chosen and precious, and whoever believes in him will not be put to shame.” So the honor is for you who believe, but for those who do not believe, “The stone that the builders rejected has become the cornerstone,” and “A stone of stumbling, and a rock of offense.” But you are a chosen race, a royal priesthood, a holy nation, a people for his own possession, that you may proclaim the excellencies of him who called you out of darkness into his marvelous light. Once you were not a people, but now you are God’s people; once you had not received mercy, but now you have received mercy. Beloved, I urge you as sojourners and exiles to abstain from the passions of the flesh, which wage war against your soul. Keep your conduct among the Gentiles honorable, so that when they speak against you as evildoers, they may see your good deeds and glorify God on the day of visitation.
The chosen race, the royal priesthood, the holy nation turns out to be those who believe in Jesus - the one about whom all the Law and the prophets testified. Most of the people standing at Sinaia that day they pledged to do everything God had spoken lied. None of them did everything God had said. Each and every single one of them eventually proved themselves to be covenant-breakers..... except for two.
However, the salient point is simple and singular: God did not ask them their opinion on anything leading up to that point and it was not until after the Christological covenant of salvation had been initiated, and its participants chosen, called, and commanded, that any explicit opportunity to choose anything was provided. Technically, God did not offer a choice in Exodus 19. It is true those people said they'd obey God, and we can reasonably infer that promise was voluntary, but that is not an episode of covenantal choice. For that group, the opportunity, the event of choice occurs much later, in Deuteronomy.
Deuteronomy 30:15-20
See, I have set before you today life and prosperity, and death and adversity; in that I command you today to love the LORD your God, to walk in His ways and to keep His commandments and His statutes and His judgments, that you may live and multiply, and that the LORD your God may bless you in the land where you are entering to possess it. But if your heart turns away and you will not obey but are drawn away and worship other gods and serve them, I declare to you today that you shall surely perish. You will not prolong your days in the land where you are crossing the Jordan to enter and possess it. I call heaven and earth to witness against you today, that I have set before you life and death, the blessing and the curse. So choose life in order that you may live, you and your descendants, by loving the LORD your God, by obeying His voice, and by holding fast to Him; for this is your life and the length of your days, that you may live in the land which the LORD swore to your fathers, to Abraham, Isaac, and Jacob, to give them.
Having already been chosen, called, and commanded they were then and only then given a choice. When scripture is examined covenantally, this proves to be the uniform pattern.
Next question:
Once the covenant is initiated and God has chosen, called, and commanded its participants without regard to their knowledge or wishes (covenantally speaking), and given them the post-establishment opportunity to choose..... What is the proof or maintenance of their choice based? Covenantally speaking, upon what is the post-establishment choice predicated upon going forward form that moment?
(Hint: the answer is not "Works" or "Obedience")
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