Look at the next verse.
Jeremiah 17:10
"I the LORD search the heart and examine the mind, to reward a man according to his conduct, according to what his deeds deserve."
God came to teach those that were made in the image and likeness of Him. There is no point in teaching those that will never be able to know, or understand what's being taught. That means folks can know their own hearts. They've been given a reference.
Buzz.
The heart of mankind is evil....until the Holy Spirit comes.