Both Isaiah and Amos use this strong metaphor of the plumbline. What does it mean when applied to the ways of Yahweh?
Read the context of Isaiah 28, look at the heading in some Bibles. It says, "Woe to Ephraim." Ephraim is another name of the Northern Kingdom, Israel. Most of Isaiah is directed to kings in Judah and Judah's citizens, but there are other oracles for Moab, Egypt, Assyria, and Ephraim. So this chapter is an oracle for the Northern kingdom, but it can apply to what God does in Judah as well, and certain can show how Yahweh deals with his people, what his standards are.
The plumbline is an ancient tool that is still used today to build a wall or find the verticality of earth's center gravity and see that something is perfectly vertical.
The people of Israel and Judah had gone crooked. They cheated people in the marketplaces with "dishonest scales." They lied in courts against the innocent. They set up a loan shark system for the poor to borrow money, but then took collateral that the poor needed, such as coats, and as Isaiah's contemporary prophet to the North, Amos, graphically points out in Amos 2:8, they took those coats the poor needed, and used them to lie down and have sex on. They sinned economically against their neighbors, lied, cheated, stole, and rapaciously sinned sexually, immorally with prostitutes of temples of foreign and false gods.
So, is there any doubt God needed to set up a plumbline of justice, a standard for them to keep. You might ask, what about the ten commandments? The 618 laws around the ten commandments? Those still applied to the Israelites and Judahites, but the people had gone so far, a straight shooter had to become the standard.
Father, in Christ you came as the ultimate standard or plumbline, walking the earth. You became the plumbline of all humans to walk straight, upright, just, honestly, sexually pure, morally sound and wisely thinking. The prophets, however imperfectly pointed to this standard, did their level best to be a standard like this, even laying their own lives on the line to be the standard. Ultimately though they pointed to the Messiah who would be this plumbline for all humanity to see and imitate: Jesus Christ the Plumbline.
Take time this week to "straighten" things around your house to get ready for a visitor. Imagine a visitor is coming who will check to see how straight things are. What if Jesus visited your house. What would he find there? Would he find media that de-humanize with violence or "casual sex"? Would he find pornography? Would he find a dusty Bible you've not read? Would he find junk food and a sloppy life or would he find order as a means to centering your life on God? Would he find healthy food, a place you pray, a Bible you read, books that feed your mind? A spirit of joy?
Give up anything in your home that you wouldn't want Jesus to find upon his visit.