There is deliberateness to all that God does. Saying that he is slow to anger is not the same as saying that he represses his anger until he cannot control it and goes amok. It means that whenever we see God angry it has been well-planned – it is never a fit of rage. This is no doubt what is alluded to in the previous verse, which in Hebrew, explicitly calls God the “master of fury.” God is the master of fury, not mastered by it. Just like a master of the tongue, that is, "eloquent." The Dutch theologian Hugo Grotius (1583-1645) describes this as, "One who, if He pleases, can most readily give effect to His fury"
There is a frightening connection made in this verse between God’s anger and his power. Many people are angry yet impotent to do anything about it. God is all powerful; he is the Almighty. When he gets angry he has all power to do whatever he wills to those with whom he is angry. This epithet, “slow to anger” is used by God of himself in Exodus 34:6 when he passes before Moses. Yet it seems to bear the opposite connotation here as it does there. In Exodus the emphasis is on God’s grace and compassion. Here the emphasis is on his justice. The most severe cases given in Scripture of God’s wrath poured out are the Flood, the destruction of Sodom and Gomorrah, and the death of Christ, all of which show deliberate action, premeditated and planned. The justice of God is highlighted particularly in the second clause of this word. It is God’s glory that he doesn’t leave the guilty unpunished. Whatever injustice may pervade the affairs of men, God will rectify all things: some of this lifetime the rest at the Judgment.
“He afterwards adds, that God possesses wrath I do not take חֵמָה, (chema), simply for wrath, but the passion or heat of wrath. We ought not indeed to suppose, as it has been often observed, that our passions belong to God; for he remains ever like himself. But yet God is said to be for a time angry, and forever towards the reprobate, for he is our and their Judge. Here, then, when the Prophet says, that God is the Lord of wrath, or that he possesses wrath, he means that he is armed with vengeance and that, though he connives at the sins of men, he is not yet indifferent, nor even delays because he is without power, or because he is idle and careless, but that he retains wraths as he afterwards repeats the same thing, He keeps for his enemies. In short, by these forms of speaking the Prophet intimates that God is not to be rashly judged of on account of his delay, when he does not immediately execute His judgments; for he waits for the seasonable opportunity. But, in the meantime there is no reason for us to think that he forgets his office when he suspends punishment, or for a season spares the ungodly. When, therefore, God does not hasten so very quickly, there is no ground for us to think that he is indifferent, because he delays his wrath, or retains it, as we have already said; for it is the same thing to retain wrath, as to be the Lord of wrath, and to possess it.” Calvin Commentary on Nahum 1:3
“He now adds, By clearing he will not clear. Some translate, “The innocent, he will not render innocent.” But the real meaning of this sentence is the same with that in Exodus 34; and what Moses meant was, that God is irreconcilable to the impenitent.” ibid
God will not acquit the wicked. Never once has He blotted out sin without punishment. Calvary proves the truth of that. The wonders of vengeance in the Old Testament and hell itself are proofs of the text. Trace this terrible attribute to its source. Why is this? God will not acquit the wicked, because He is good. Goodness itself demands the punishment of the sinner. The justice of God demands it.
Earlier we mentioned the scoffers of 2 Peter 3:4, “Where is the promise of His coming?” and Peter’s response that the slowness in punishing the evil that we may be tempted to ascribe unto God is actually for our benefit because God is waiting until everyone of His elect come unto Him: He is patient toward us, this is the reason for the imaginary delay in punishing the wicked.
The larger context, i.e., 2 Peter 3:4-9 says: This is now the second letter that I am writing to you, beloved. In both of them I am stirring up your sincere mind by way of reminder, that you should remember the predictions of the holy prophets and the commandment of the Lord and Savior through your apostles, knowing this first of all, that scoffers will come in the last days with scoffing, following their own sinful desires. They will say, “Where is the promise of his coming? For ever since the fathers fell asleep, all things are continuing as they were from the beginning of creation.” For they deliberately overlook this fact, that the heavens existed long ago, and the earth was formed out of water and through water by the word of God, and that by means of these the world that then existed was deluged with water and perished. But by the same word the heavens and earth that now exist are stored up for fire, being kept until the Day of Judgment and destruction of the ungodly. But do not overlook this one fact, beloved, that with the Lord one day is as a thousand years, and a thousand years as one day. The Lord is not slow to fulfill his promise as some count slowness, but is patient toward you, not wishing that any should perish, but that all should reach repentance.
We should note that God’s “patience” is not with the wicked; it is with the elect, whom Peter addresses as “you.” God’s attribute of being longsuffering is never directed at the ungodly. It is always directed at His covenant people. The wicked who aren’t punished instantly merely benefit from it, of you can call delayed damnation a “benefit.”
In an essay called “The Scriptural Presentation of God’s Hatred,” published in 1967, Homer C. Hoeksema writes, “We must understand, in this connection, that the Word of God throughout draws a very sharp line of demarcation between the godly and the ungodly, the righteous and the wicked, the church and the world, the children of light and the children of darkness. We must not fall into the error that is so common today, and that sounds so piously evangelistic, of speaking simply of the ‘unconverted,’ conceiving then of the unconverted from our own point of view, not as God sees and knows and views them. When Scripture makes the distinction between the godly and the ungodly, it always refers principally to the ungodly that will never be converted, the ungodly that will persist in his ungodliness until he is cast into everlasting destruction. Nor must we say that we cannot very well make these the object of our contemplation for the simple the reason that we do not know who they are. For, in the first place, while it may be true that the individual identity of the ungodly to a certain extent may belong to the realm of the secret things, nevertheless the fact that there are such men belongs to the revealed things of God. This must be reckoned with, both as far as the preaching of the gospel is concerned, lest the preacher delude himself that all men are potential converts, and as far as the life and calling of God's people in the midst of the world are concerned, lest they make common course with the wicked. In the second place, we must not forget the principal truth: by their fruits ye shall know them. And, in the third place, the question is not whether we in every case can individually distinguish the ungodly, but whether God knows them, and what is His attitude toward them. And then the fact is that God does not have before His divine eyes a mass of unconverted men who are possible candidates for conversion; but there are before Him the righteous and the unrighteous, the godly and the ungodly,—two distinct classes of men.”
All of this simply means that reprobation serves election. As Lorraine Boettner put it:
This decree of reprobation also serves subordinate purposes in regard to the elect; for, in beholding the rejection and final state of the wicked,
(1) they learn what they too would have suffered had not grace stepped in to their relief, and they appreciate more deeply the riches of divine love which raised them from sin and brought them into eternal life while others no more guilty or unworthy than they were left to eternal destruction.
(2) It furnishes a most powerful motive for thankfulness that they have received such high blessings.
(3) They are led to a deeper trust of their heavenly Father who supplies all their needs in this life and the next.
(4) The sense of what they have received furnishes the strongest possible motive for them to love their heavenly Father, and to live as pure lives as possible.
(5) It leads them to a greater abhorrence of sin.
(6) It leads them to a closer walk with God and with each other as specially chosen heirs of the kingdom of heaven.
(7) In regard to the sovereign rejection of the Jews, Paul destroys at the source any accusation that they were cast off without reason. “Did they stumble that they might fall? God forbid: for by their fall salvation is come to the Gentiles, to provoke them to jealousy,” Rom. 11:11. Thus we see that God’s rejection of the Jews was for a very wise and definite purpose; namely, that salvation might be given to the Gentiles, and that in such a way that it would react for the salvation of the Jews themselves. Historically we see that the Christian Church has been almost exclusively a Gentile Church. But in every age some Jews have been converted to Christianity, and we believe that as time goes on much larger numbers will be “provoked to jealousy” and caused to turn to God. Several verses in the eleventh chapter of Romans indicate that considerable numbers are to be converted and that they will be extremely zealous for righteousness.
Not to us, O LORD, not to us, but to your name give glory, for the sake of your steadfast love and your faithfulness! Why should the nations say, “Where is their God?” Our God is in the heavens; he does all that he pleases. (Psalm 115:1-3 ESV)