The Lord God has shown Amos a vision of locusts and one of fire. "Can we survive?" Amos asks.
Then the Lord shows him a plumb line--a way of measuring if a wall has been built straight. It hasn't been.
This vision in some ways is more ominous than the ones of locusts and of fire. We plant crops but we can't keep the locusts away. Some steps for fire prevention are possible, but some fires can't be stopped.
But, we could have built a better wall.
How much of the pain we suffer is due to our not following God's way for us?
The lectionary pairs this reading with the one from Mark about how upset Herod was with Jesus and John the Baptist. Could Herod have been eying the walls he was responsible for and fearing that God was about to show up with a plumb line?