Jeremiah 6:27 in the King James Version
I have set thee for a tower and a fortress among my people, that thou mayest know and try their way.
Context
26 Daughter of my people, clothe yourself with sackcloth, and wallow in ashes! Mourn, as for an only son, most bitter lamentation, for the destroyer will suddenly come on us.
27 “I have made you a tester of metals and a fortress among my people, that you may know and try their way.
28 They are all grievous rebels, going around to slander. They are bronze and iron. All of them deal corruptly.