Reprobation is the flip side of the doctrine of election. Just as God, out of His free and sovereign grace, has ordained some to mercy, out of His sovereign holiness and justice, He has ordained the rest to perdition for their sins. While many Evangelicals balk at this doctrine, it certainly isn't because of any lack of biblical support for it, especially in the Old Testament!
Consider Deuteronomy 2:30, "Sihon the king of Heshbon would not let us pass by him, for the LORD your God hardened his spirit and made his heart obstinate, that He might give him into your hand, as he is this day." God had purposed the destruction of Sihon and his people, so He hardened the king's heart.
And Joshua 11:19-20, "There was not a city that made peace with the people of Israel except the Hivites, the inhabitants of Gibeon. They took them all in battle. For it was the Lord’s doing to harden their hearts that they should come against Israel in battle, in order that they should be devoted to destruction and should receive no mercy but be destroyed, just as the Lord commanded Moses." Here we see the same act of hardening, but over a larger area.
Or the case of the son's of Eli, I Samuel 2:25, "'If someone sins against a man, God will mediate for him, but if someone sins against the LORD, who can intercede for him?' But they would not listen to the voice of their father, for it was the will of the LORD to put them to death." Eli tried to warn them about their evil ways, but God had already determined on their destruction, so He hardened their hearts against the words of their father.
Arminians resist the significance of these passages. They hold that God only hardened the hearts after the respective people had already hardened their own hearts, i.e., what is known as "judicial hardening." However, that appears nowhere in these passages, except in the case of the sons of Eli. Rather, the Arminian commits the very error that Paul refutes in Romans 9:21: "Has the potter no right over the clay, to make out of the same lump one vessel for honorable use and another for dishonorable use?" As Creator and Lord, God has a sovereign right to use His creatures as He sees fit. Our only option is to bow our heads and bless the Lord for His justice.
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