God showed Amos a swarm of locusts that he prepared to punish Israel. Amos, a citizen of the rival kingdom of Judah, begged him to be merciful. God relented. Then he showed Amos a consuming fire. Again Amos begged for mercy and God relented.
But then God showed Amos a wall, and next to the wall, a man with a plumb line. Amos could persuade him not to destroy the apostate kingdom with locusts or fire, but God would not allow his prophet to dissuade him from punishing the sins of his people.
King Jeroboam II had built a prosperous and militarily powerful kingdom, but he refused to heed Amos’ words. God would destroy Jeroboam’s dynasty, and then the kingdom itself, along with all of the idols the kings of Israel had erected and the sanctuaries that housed them.
A plumb line will always show a true vertical. A wall, no matter how massive and strong it looks, that has shifted from a true vertical is in danger of collapse. If there is no way to repair it, it is safer to demolish it than to let it collapse.
The northern kingdom shifted away from true vertical when its first king rebelled against Solomon’s son and set up idols so his people would not have to go to Jerusalem to worship. God raised up one dynasty after another, but none turned from that sin.
If the kingdom would not repent, God would not overrule its own free will to repair it. Instead, he demolished it. Amos could intercede on their behalf and get God to change his mind on the means, but could not halt the destruction God intended.
God still uses the same plumb line today: his word. Everything stands or falls in comparison to Jesus, the living word. God will uphold, repair, or demolish every person and every human institution according how it lines up with Jesus.
As always, grace follows judgment. God punishes sin severely, but only as a means to redemption.