What is a plumb line in the Bible?

The simple contrivance of a lump of lead, a stone, or other weight attached to a string, for testing whether a building or other erection is perpendicular. It is used symbolically for the exactness with which judgement was brought upon Israel. Israel had been built up by God as a wall with a plumbline, and with a plumbline it should be destroyed. Amos 7:7-8: cf. 2 Kings 21:13; Isa. 28:17. In Zech. 4:10, although it was a day of small things when the temple was rebuilt, the plummet was in the hands of Zerubbabel, and the Lord of hosts was supporting him.

Plumb line

A plumb line‑a string with a pointed weight on the end‑is still used today by masons to make sure they lay a brick wall straight and true. In a short little book tucked among the minor prophets of the Old Testament, God told Amos, "Look, I am setting a plumb line among my people" (Amos 7:8).

As the Holy Spirit directed Amos's thoughts, the analogy of a plumb line came to his mind and he referred to this familiar tool to tell the Israelite people what God wanted them to know - that God would measure them by His standards, His Word, and no other.

In Amos 7 a gradation of three judgments on Israel is set forth: first (verses 1-3) by the grasshoppers or creeping locusts, next (verses 4-6) by fire, and lastly (verses 7-9) by a plumbline, which intimated the strict measure applied to mark their iniquities; when patience had exhausted itself, further delay would have been connivance in evil. These troubles were accomplished historically, it would seem, in Pul, Tiglath-pileser, and Shalmaneser, who finally swept away the kingdom.

He saw the Lord standing upon a wall with the plumbline to see if the wall was straight. The test by God's Word and God's holy law shows that all is crooked and must be condemned. Therefore, the announcement, "I shall pass by it no more. And the high places of Isaac shall be desolate, and the sanctuaries of Israel shall be laid waste; and I will rise against the house of Jeroboam with the sword." The false worship and the monarchy in Israel will be completely swept away by the judgment.

Amos 7. God had long waited patiently. More than once He had been on the point of giving Israel up to judgment. The intercession of the prophet, that is to say, of the Spirit of Christ which wrought in the prophets (an intercession, indeed, that owed its efficacy to His sufferings; see Psalm 18), had arrested the scourge. But now Jehovah would arise to judgment, with the measuring-line in His hand, and nothing should turn Him aside. With the house of Jehu Israel should fall. In fact this is what took place. It may be that the preceding judgments apply to the downfall of the family of Jeroboam, the son of Nebat; and to that of the family of Ahab. Israel had been raised up again after each of those events, but not so after the house of Jehu had fallen.

We needed a wall outside our house, and I called a mason to build it. He made a very poor foundation, and I said to him, Your wall will not stand up. However, he guaranteed it, and insisted on his own way. Soon the wall appeared to me to be leaning, but I was not sure, so I got a plumb line. I found that the wall was more than a foot out of plumb. I called for the mason. He looked at the wall, and said, "That wall is all right. It is quite straight." I did not say a word, but hung the plumb line on it. The plumb line showed how far from true was the wall. The mason's mouth was shut, he could not say a word; his pride and self-confidence were clearly shown forth. But it was necessary to use the plumb line in order to do this. Just so, it was necessary to use the law, in order to show forth man's pride and self-confidence. But the plumb line could not make the wall straight; it could only show how bad it was. So the law cannot make us righteous, but only show how bad we are.

The law shows up the crookedness of man, that “sin might appear exceeding sinful.” It has often been illustrated by the plumb-line which is put against a crooked wall. It shows up its crookedness, but does not rectify it. So the law shows up the crookedness and sinfulness of man, but does not put right what is wrong. Those, however, who see their sinfulness and own it to God, seeking His mercy, are justified freely through the redemption which is in Christ; Jesus.

But what do we really find? The law is like a plumb-line, which, showing the crookedness of the wall, has no power whatever to make the wall straight. One, who most earnestly sought to keep the whole law, and who was blameless in his outward life, had to confess, “The commandment, which was ordained to life, I found to be unto death” (Rom. 7:10). Nor was this alone the experience of Saul of Tarsus, afterwards known as the Apostle Paul.

Law cannot justify, nor can it sanctify. It is God's plumb-line making manifest man's crookedness; His mirror shewing up his vileness.

219 In the judgments of God, the question one sees debated is between the rights of Satan because of sin, and those of God. It is necessary that the people of God should be judged on the one hand and on the other, that in the midst of the judgment salvation should be found, as Noah in the ark was carried over the waters of the deluge. It is in the cross of Christ that faith sees the judgment of God against our sins, and for ourselves; we are thus saved righteously. So here also judgment will Jehovah lay to the line, and righteousness to the plummet, when He lays in Zion for a foundation a stone, a tried stone, a precious corner stone. The hail shall sweep away the refuge of lies, the waters overflowing the hiding place; but the Stone abides sure and stedfast. The very same thing - God's judgment - which destroys the wicked guarantees the believer against all evil, for he is made God's righteousness in Christ.