Not knowing that the goodness of God leads you to repentance … in accordance with your hardness and your impenitent heart you are treasuring up for yourself wrath in the day of wrath and revelation of the righteous judgment of God.
If God gives us proofs of His goodness, it does not mean that He simply overlooks our sins. If He does not punish evil immediately, but shows patience and long-suffering, it does not mean that judgment will not ensue. God has a purpose in all this: to lead us to repentance.
In the context of these verses the very opposite of repentance is revealed among the “decent” persons addressed here. They wish to justify themselves. Comparing themselves with others and their sins, they imagine that they come off well. They fail to acknowledge God’s judgment on them and do not dream of changing their way of life. That is precisely the stubborn reaction of an “impenitent heart”.
In biblical language “repentance” is quite the contrary. When someone repents, he ceases glossing over his guilt, regrets it and acknowledges God’s verdict on it; he no longer considers himself righteous, but admits that his sins stem from an evil heart; he no longer justifies himself but God (cf. ch.3:v.4); he alters his way of thinking, which becomes visible in a change in his way of living.
For Paul repentance is an essential component of his preaching: “testifying to Jews, and also to Greeks, repentance toward God and faith toward our Lord Jesus Christ” (Acts 20:21).