The plumb line metaphorically represents an external standard to distinguish right from wrong. Most likely it is a reference to the first five books of the Bible (Genesis to Deuteronomy) which embodies God’s self-revelation and covenant with his people and also the standard against which Israel has been found wanting.1
8 And the LORD said to me, “Amos, what do you see?” And I said, “A plumb line.” Then the Lord said, “Behold, I am setting a plumb line in the midst of my people Israel; I will never again pass by them;