Charles Spurgeon Commentary Jeremiah 3:9

"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 3:9 (ASV)

And it came to pass through the lightness of her whoredom that she defiled the land, and committed adultery with stones and with stocks.

That is to say, she gave her heart to false gods, and worshipped stones and stocks. And how it must anger the living God to see people turn away from Him to worship blocks of wood and stone instead of Him, and especially a people who have been instructed concerning the living God, and so commit the grossest act of disloyalty to Him, and are rebellious to the utmost degree.