‘The sin of Judah is written with a pen of iron’, not so much a pen as an implement in the original. It could be an engraving tool, but more than a pen.
We learn here how powerfully idols take hold of the heart, the affections, the emotions; they take them over and rule them. Much of the book is against idolatry, but they are not listening. The people of Judah are not paying attention to the warning that came through the fall of Samaria, in which the ten northern tribes also became prey to idols, and were eventually removed from their land.
Now the idol represents anything which is believed in and trusted and relied on as an alternative to Almighty God. The worship in which they were trained, the worship that had been given through Moses was rejected by them. They followed it notionally, nominally, but it didn't mean anything to them, and they made their own idols on the hilltops.
Of course, the same applies to Christians. Those who love the Lord, can fall into a form of idolatry where they trust in human skills and things and innovations, and love them, and trust in them rather than in the Lord, and human help. Any form of idolatry is the greatest possible abomination and offence to God. And yet society is lost in it, and we too were once lost in it, and sometimes as Christians we return to it in some measure. There is something that we love far too much, or depend on far too much, and we begin to be offensive in the sight of God, and he may have to chastise us in anger. It is a horrible insult to him to be so attached to something in this world, to need it to lift our spirits and make life worth living, to turn to that thing or that experience as if it were far more powerful than the Lord. The one who worships idols loses the ability to find pleasure in the Lord.