Charles Spurgeon Commentary Jeremiah 3:8-9

"And I saw, when, for this very cause that backsliding Israel had committed adultery, I had put her away and given her a bill of divorcement, yet treacherous Judah her sister feared not; but she also went and played the harlot. And it came to pass through the lightness of her whoredom, that the land was polluted, and she committed adultery with stones and with stocks." — Jeremiah 3:8-9 (ASV)

And I saw, when for all the causes whereby backsliding Israel committed adultery I had put her away, and given her a bill of divorce; yet her treacherous sister Judah feared not, but went and played the harlot also. And it came to pass through the lightness of her whoredom, that she defiled the land, and committed adultery with stones and with stocks.

Bowing down before idols made of wood or stone.