"You have the forehead of a whore; you refuse to be ashamed" (v. 3b).
The prophets of Israel and Judah frequently used vivid imagery to shock readers and help them see their sin for what it truly was. Today's passage is one example of this. Jeremiah accuses Judah of committing adultery by abandoning God for idols, and asserts that the Judahites "have the forehead of a whore" (Jer. 3:3b). Judah's idolatry was a way of life. The people were not secretive, but advertised their debauchery just like a prostitute.
The prophet then tells the people that they "refuse to be ashamed." Sin is supposed to bring with it feelings of shame, for we have been made in God's image and so feel the awfulness of what we have done, at least in some respects. But, there comes a point when we can have a seared conscience, when we have sinned and have so long ignored the voice within telling us that we have done wrong that we no longer hear it. This was the status of the majority of Judahites in Jeremiah's day—they were not ashamed of their sin any longer because they had been ignoring the prophets and even their own consciences for years.
As we have noted, the objective reality of our guilt and our guilt feelings do not always line up. The conscience can get seared, so we must return again and again to the Word of God so that our consciences are formed by His truth through His Spirit. Faithful preaching of the Word is God's means to heal seared consciences and keep them tender to sin (Acts 2:37).
Not only can we be guilty and yet not feel guilty, but there are times when we feel guilty but are not objectively guilty before the Lord. Often, we find this in cases where we have been taught that something is a sin when it is, in fact, not a sin. For example, many people have been told that to drink any alcohol at all is a sin. Scripture, however, never teaches this. It affirms that drunkenness is a sin, but consuming alcohol in moderation is no vice (1 Cor. 6:9–10). In fact, the Bible says wine was given to make the heart glad (Ps. 104:14–15).
We are not automatically guilty before God for doing what He does not forbid. Yet there is complexity here due to the Bible's teaching that what is done contrary to faith is sin (Rom. 14:23). We sin if we act in a way that we sincerely believe is wrong even if the Lord has not called it evil. The guilt is not for the act itself but for acting against faith. If one wrongly believes something to be a sin and does it anyway, he is nevertheless committing sin because he is living as if God's standards (however mistakenly understood)—and thus God Himself—do not matter to him. He's showing no regard for the Lord as the holy lawgiver.
Coram Deo
Our guilt feelings will fully line up with the objective reality of our guilt only insofar as our consciences are properly shaped by the Word of God. Scripture informs our consciences, helping them to more quickly and more correctly discern good from evil. This helps keep the conscience from becoming seared, but it also helps us overcome false notions of objective guilt that come from calling something sin that God has not defined as sin.