Charles Spurgeon 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)

The sin of Judah is written with a pen of iron, and with the point of a diamond:

It is so ingrained in their very nature that you might as well try to erase an inscription that is written upon steel with the point of a diamond as hope to get this perversity out of the nation; it is graven upon the tablets of their heart. What is mere habit can be altered, but what is ingrained in the heart cannot be taken away except by a miracle of grace. It was the heart that was wrong; the fountain-head was polluted, so what could the streams be but foul.

It is graven upon the table of their heart, and upon the horns of your altars;

Their holiest things were defiled. They wrote up the names of their idol gods even upon God's altar, and so they bore a written testimony against themselves.