Albert Barnes Commentary Jeremiah 17:1

"The sin of Judah is written with a pen of iron, [and] with the point of a diamond: it is graven upon the tablet of their heart, and upon the horns of your altars;" — Jeremiah 17:1 (ASV)

This section (Jeremiah 17:1–4) is inseparably connected with the preceding. Judah’s sin had been described (Jeremiah 16:19) as one of which the very Gentiles will become ashamed, and for which she will shortly be punished by an intervention of God’s hand more marked than anything in her previous history. Jeremiah now dwells upon the indelible nature of her sin.

A pen of iron - that is, an iron chisel for cutting inscriptions on tablets of stone.

The point of a diamond - The ancients were well acquainted with the cutting powers of the diamond.

Altars - Not Yahweh’s one altar, but the many altars which the Jews had set up to Baalim (Jeremiah 11:13). Though Josiah had purged the land of these, yet in the eleven years of Jehoiakim’s reign they had multiplied again, and were the external proofs of Judah’s idolatry, as the table of her heart was the internal witness.