John Chrysostom
Having praised God's long-suffering and shown its greatest benefit for the attentive, namely, that it draws sinners to repentance, the Apostle increases fear. For those who use it as they should, it serves as the foundation of salvation. For those who despise it, it becomes an occasion for greater punishment.
As for the widespread opinion that God, being good and long-suffering, does not seek punishment, the Apostle suggests this: when you say it, you speak of nothing other than bringing punishment upon yourself.
For God shows His greatest mercy so that you may be freed from sins, not so that you may add new ones. As soon as you do not do this, the punishment will be more dreadful.
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Since God is long-suffering, for this very reason you especially must not sin or turn His benefits into an occasion for ingratitude. Although He is long-suffering, He nevertheless also punishes. Whence is this clear? From the Apostle's following words.
If impiety is great and the impious have remained unpunished, then it is wholly necessary that they be subjected to punishment. If even men do not leave this without attention, how will God leave it? Thus from here the Apostle began to speak about judgment.
Having proved that many, if they do not repent, are found guilty, and then that they are not punished here, he concludes from this that there must be a judgment, and a very severe one. Therefore he says: "But according to your hardness and impenitent heart, you are storing up wrath for yourself."