Evidently, Amos' teaching was effective because the people responded - at least it caused a reaction. He was a good strategist; he preached at the shrines where the people were. His influence radiated out as the word spread that a prophet from Judah was proclaiming doom for the nation. The people listened and spoke to each other about his preaching. When Amos accused the religious leaders of Israel of failing to teach God's way of life, Amaziah, a high religious official of the shrine in Bethel, felt he needed to respond.
As we see in Amos' case, a person can obey God and still receive public persecution. God will not protect us from all persecution, partly because it affords an opportunity to witness for and glorify Him. Amos' answer to Amaziah's charges makes this witness and enables him to prophesy further. Additionally, his response instructs us regarding the nature and function of a prophet.
This also shows a clear example of the biblical use of a plumb line, a building tool used to determine if an object is upright (verses 7-9). Does God hold the plumb line against Amaziah or Amos? Actually, He judges both. Amaziah represents the false religions, and Amos represents the true religion. The content of their conversation reveals how God would judge them. Primarily, though, God was evaluating Amos.
We need to apply the plumb line to ourselves. Are we taking the grace of God for granted? Could God be angry with some of us in His true church? Revelation 3:14-22 shows that the Laodiceans are sincere when they assert that they are spiritually complete, but God is ready to vomit them out! Obviously, the Laodiceans are not judging themselves against God's plumb line, or they would have known they were out of alignment with His will.
Because they feel so secure in their own spirituality, they probably think it incredible that God would single them out for punishment. It is clear, however, that God punishes those who forsake their part of the covenant with Him. Revelation 12:17 shows that, on the other hand, Satan persecutes those who keep the commandments of God and live godly lives.
God's religion is more than keeping the basic Ten Commandments. The Pharisees kept them, but our righteousness has to exceed theirs (Matthew 5:20). One difference between Christ and the Pharisees was that Christ's righteousness was positive while the Pharisees' was negative. Though both kept the commandments, the sincere Pharisee was righteous by avoiding sin, but Christ was righteous by always doing good as well.
The problem of the Laodicean is selfishness, self-concern. His opposite, the Philadelphian (which means "brotherly love"), is commended by God for his obedience and for doing good. His religion is outward in practice because he has prepared himself to give and serve through his relationship with God. The Laodicean is too busy gathering his wealth and indulging himself to give much thought to his fellow man.
Like the Laodiceans, the ancient Israelites concentrated on self-advantage, self-pleasing, and covetousness. This resulted in their being very hard on the needy and the poor. They ignored doing good works and serving their brothers. Amaziah apparently felt he needed to speak out and defend "that old-time religion."