A plumb line, also called a plummet, is a cord with a non-magnetic weight attached to one end. When the cord is held in such a way that the weight can dangle freely, an exact vertical can be determined. Painters and carpenters use plumb lines to keep their work straight. It is difficult, while in the middle of a project, to determine a true horizontal or vertical line without an objective measuring tool, so a plumb line is employed. A plumb line applies the law of gravity to find right angles, to indicate the most direct route from top to bottom, and to keep things plumb. A plumb line doesn’t change or move with the whims of the carpenter. It remains true, and all work must line up with it or risk being crooked.
The term plumb line is used in Scripture in several contexts. The Lord pictures Himself as a builder in Isaiah 28: “See, I lay a stone in Zion, a tested stone, a precious cornerstone for a sure foundation” (Isaiah 28:16). This is a promise of an unshakeable kingdom, with the Messiah in charge. Jesus Christ is the “precious cornerstone” (see Mark 12:10; Acts 4:11; Romans 9:33; 1 Peter 2:6). As the Lord builds His kingdom, He will ensure it is perfect in every way: “I will make justice the measuring line and righteousness the plumb line” (Isaiah 28:17). There will be no crookedness/sin in the kingdom of Christ.
When the Jews returned from captivity in Babylon, they began to rebuild the temple in Jerusalem. For a time, the work languished, and God sent the prophets Zechariah and Haggai to spur the people on to finish the rebuilding. The message was encouraging: do not despair over the small beginnings; God will see to it that the temple is completed, and Zerubbabel the governor will oversee the project. “Whoever has despised the day of small things shall rejoice, and shall see the plumb line in the hand of Zerubbabel” (Zechariah 4:10, ESV).
Amos 7:7–8 says, “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.’” When God said He was setting a plumb line among His people, He was declaring an end to their attempts to justify their crooked ways. The Lord was setting the standard. God does not negotiate His laws. He does not change with the whims of culture (Numbers 23:19). God’s moral law is the plumb line against which we determine right and wrong (John 17:17). Just as a carpenter’s plumb line is not subject to the opinions or the frustration of the worker, so God’s moral standards are not subject to the opinions of man. Wise people are those who line up their lives according to God’s plumb line rather than trying to move it to satisfy their own agendas.