Amos sees the Lord standing by a wall built true to plumb (vertical), and the wall is built true to plumb because the Lord built the wall using the plumb-line that He held in His hand. The wall symbolizes God's remnant people, that wall of living stones that He is building up into a holy habitation for the Lord. It speaks of the walls that surround Jerusalem that have been broken down, and that He is restoring.
The Lord asked Amos what he saw, and Amos answered, "A plumb-line." This plumb-line that Amos saw is the word of God. However, when he saw the word of God, he saw it not as the word of God, but as a plumb-line. He saw it as the standard of truth that it is. He saw it as the standard for vertical against which all structures would be judged as true. Just as the gravity that hangs a plumb-line perfectly and only vertical at all times is one of the most immutable, consistent, and unavoidable physical forces known to mankind; a force so powerful it actually holds the planets of our solar system in their orbits, so the eternal Character of God Himself is the force behind the standard of His word. His word is the standard against which all spiritual buildings are compared, and it is the very means by which He builds a spiritual building. His word is the plumb-line He uses to build a plumb spiritual wall. However, it is only seen as a plumb-line in relation to the wall. In order for Amos to see the plumb-line, the Lord held it up next to the wall.
Just as Amos looked at God's word and saw a plumb-line next to a wall, so the standard of God's word will only be revealed as the only standard of plumb as it is seen in relation to His holy remnant people. God is going to hold His word up to His people and say look, the wall matches the plumb-line - My people match my word! Only in relation to His people can anyone truly see the standard of His word, for when the church at large views the word of God they see it not as a plumb-line, but as a string of concepts threaded to the weight of Biblical authority. They can overlook much or twist much in order to fit their concepts of God or of spiritual things. They can tear it down to the level of their meager experience. However, when the word of God is held up next to those who have been built according to its standard, suddenly, all else is shown to be crooked. Suddenly, in relation to God's people who are becoming the word of God, the true standard contained in His word is revealed, and the poverty of the church's religion is revealed.
God is building a spiritual building. He is rebuilding the walls that have been broken down since the Apostles and Prophets of old set the standard. When that wall is completed, He is going to hold His holy and immutable word up to his glorious remnant church, and His people will meet the standard of His word. He is then going to set the plumb-line in the midst of His people Israel (the carnal masses of Christendom). He is going to take that plumb-line that is shown to be the standard of plumb by the people it built and to whom it was raised, and is going to hold it up to his people who refused its work, and He is going to irrefutably show for all of heaven, hell, and Earth to see that she does not match up to the standard of His word. He will once and for all show the poverty and the fantasy of a people who believed a lie about themselves, seeing themselves as rich and increased with goods, and in need of nothing, when in fact the standard will show them to be wretched, pitiful, poor, blind and naked.
God said, "Behold, I will set a plumb-line in the midst of my people Israel; I will not again pass by them any more..." God is going to build His remnant people up until they match the standard of His word, and He is then going to hold His word up next to them to show that they match up. His people will thereby show the word of God to be the standard by which God will measure His people, and God will then set the standard up against Israel sparing them no longer.
What a terrible revelation when the religious church sees for the first time what God has tried so many times to show her. She will see her true condition. She will for the first time see what His remnant church has had to face so many times: her true condition. Because His remnant church admitted her true condition that she wasn't built plumb, the line could be used to build her plumb. The word of God was free to do its intended work. Not so with Israel. She refused to admit her true condition, and now, oh the horror of the revelation that she's not a part of that spiritual building at all. Oh, the terrible revelation of her true condition only after it's too late. For her, the line will no longer be used to build her but to show the shame of her condition. He will spare her no longer but will set His word in her midst for judgment.
As He built His holy people up into a spiritual building, He held her up to judgment over and over. She wept and mourned over her condition over and over, crying out to God to change her, to grant repentance, to take away her poverty and her bareness. In that day, her tears will be dried as she weeps no more. Her mourning will be turned to dancing. However, for Israel, for the carefree city that dwelt at ease, that said in her heart I am and there is no other, to her will be given confusion of face, weeping, and mourning. Her laughter will be turned to tears as the Lord comes to her with the plumb-line in his hand.
The Lord is going to come to His people - all of His people. He will draw near to His people, but as He says in Mal. 3:5 speaking of those for whom it is too late, "Then I will draw near to you for judgment..." The Lord told us years ago that He is coming to every church in America that He knows will open the door to Him when He knocks. For some He will come for blessing, and for some He will come for Judgment - the people will decide which. He is coming with His plumb-line in hand.