One of the questions Amos forces us to ask is what makes a nation, a community or individual righteous? How does God measure righteousness? This is the lesson in the plumb line.
This is what he showed me: The Lord was standing by a wall that had been built true to plumb, with a plumb line in his hand. And the LORD asked me, “What do you see, Amos?” “A plumb line,” I replied. Then the Lord said, “Look, I am setting a plumb line among my people Israel; I will spare them no longer.