Wrong argument
4 Gen. 14: 18-20 18. The Lord slow to anger and full of great mercy, forgiving iniquity and crime, who does not acquit the guilty, but visits the iniquity of the fathers on their sons to the third and fourth generations. 19. Forgive, please, the iniquities of your people according to the greatness of your mercy, as you have forgiven this people from Egypt until now. 20. Then the Lord said, I have forgiven according to your word.

4 Gen. 14:23 They will not see the land that I have sworn to their fathers, and none of those who provoked me will see it.

4 Gen. 14:35 I, the LORD, have said that I will do this to all this wicked congregation that has conspired against me; in this desert they will perish and die here.

The above passages from the   Book of Numbers are used by some to teach God's mercy to condemned persons. The argument is as follows:

“Moses prays that God would forgive Israel and not destroy it completely after 10 spies brought bad news. God forgives Israel according to the greatness of His mercy, although they are a bad congregation (and will remain evil until a whole generation dies in the desert). "

Note that according to this concept:

God's forgiveness for these damned persons is not salvation (although at this point they were spared from God's judgment, all ultimately died in the desert);
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Moses in his prayer claims that God is merciful and forgives even the damned by nature.

Absolute forgiveness
This is a very important point. Not only here in the Book of Numbers , but also in many other passages, it seems that the Lord is simultaneously promising salvation and threatening the same people with eternal judgment . Of course, this cannot be the case. He cannot forgive people and send them to Hell. If the Lord would forgive all who sinned in Kadesh by rejecting the accounts of Joshua and Caleb and refusing to enter Canaan, they would not have perished in the wilderness.

For the same reason, since they died in the wilderness under God's judgment, they were not forgiven . The forgiveness of sins is absolute. If God forgives someone's sins, He has justified that person, and there is no way that person will perish under His wrath.

Nor is there such a thing as "unforgivable" forgiveness. This is the same as saying that

there is a forgiveness that does not forgive and
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salvation that does not save.
If a judge pardons me, I am free from all legal consequences of the crime I committed and can never be charged with that crime again. If I am sent to prison or executed for my crimes, I have not been pardoned.
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Judgment of the damned
Neither delaying judgment is a kind of forgiveness, but on the contrary, because the unrepentant sinner has more time to sin.

Rome. 2: 5 But by your hardness and unrepentant heart you stockpile yourself with anger for the day of God's wrath and the revelation of God's righteous judgment

If delay in court is a kind of forgiveness, then God has spent six thousand years forgiving those he intends to destroy eventually. If a judge delays my sentence for an offense by setting a different date of conviction, this is not a pardon in any sense of the word, but merely a delay.

That God is merciful "by nature" is true, but He is not so for the damned or for those who perish. If this were true, he would deny himself, deny his own righteous nature to punish them forever.
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Institutional Church
This does not answer the question of how God can speak of forgiveness and judgment in almost the same breath to those who have sinned. The answer is that God does not speak to the individual, but to the nation, to the Old Testament church.

Happening. 7:38 He was in the assembly in the wilderness with the angel that spoke to him in Mount Sinai, and with our fathers. He accepted God's living words to convey to us.

This nation, the Old Testament church, like the New Testament church, is always a mixed crowd. They are in the church

those whose sins are forgiven forever
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but there are also those who perish without forgiveness under the judgment of God.
Because they are mingled with each other, God's Word, both His promise of forgiveness and His threat of eternal damnation, reach all, though the promises are intended solely for the benefit of those who have been chosen by God and redeemed by the blood of Christ.
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Spiritual Israel
This is the teaching in  Romans

Rome. 9: 6-7 6. But it is not possible that the word of God should fail. For not all who come from Israel are Israel . 7. Not all of them are children because they are Abraham's offspring, but it is said: In Isaac your offspring will be named.

Note that Paul insists that God's Word cannot fail, in this case neither His word of forgiveness nor His word of judgment. His word of forgiveness does not fail when he promises forgiveness to Israel, because those who only have the name Israel are not really the Israel addressed by God. It also does not fail because "the children of the promise are counted as offspring ," counted not only among the true children of Abraham , but among God's children, whom He always forgives in His love through the cross.

Rome. 9: 8 That is, it is not the children of the flesh that are children of God, but the children of the promise are considered seed.

In the institutional Church there are always those who worship together with believers and who, in many cases, cannot even be distinguished from them, but who are not really the Church which is the Body of Christ, "the fullness of him who fills all in everything" , the Church, who obtains peace and forgiveness in Him, while those who remain hypocrites and unbelievers in the Church never receive it.

Eph. 1:23 Which is his body and  the fullness of him who fills all in all.

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Rome. 11: 7-8 7. Well then? What Israel seeks, it has not achieved, but the elect have achieved, and others have been plunged into hardening ; 8 .  (As it is written: God gave them a spirit of hard sleep, eyes not to see, and ears not to hear) until this day.

A fragrance for life and death
Does this mean that the word of forgiveness does not apply to those who are perishing? No, it doesn't . Those who perish must hear God's word of forgiveness to their own damnation . God is "by nature" a merciful God, and the proclamation of His mercy makes all those who hear His mercy and do not believe guilty before Him and most worthy of His righteous judgments.

2 Cor. 2: 15-16 15. For we are the sweet fragrance of Christ to God among those who are saved and among those who are perishing. 16. For some it is the fragrance of death to death, and for others it is the fragrance of life to life. But who is fit for this?

Does this mean that God's word of judgment does not apply to those He forgives? Not! The word of judgment must be heard by those who are forgiven, not because they will ever be subject to God's eternal judgment (thank Him for the gift of His Son!), But because they too have sinned and must repent and turn from their sins. as they always do by the irresistible grace of the Spirit (hence the questioning of human repentance: But who is fit for this? )

The word of God's forgiveness and His judgment for sins reaches out to all who hear the Word, and the same Word sifts,  hardens , and judges those who have only the name Israel or the Church, bringing peace and forgiveness to those who, according to His eternal choice, by the blood of Christ and by the action of the Spirit, they belong to God.