A major proof of false religion is that it cannot validate its effectiveness before the witness of man, but God can and does validate the true religion. He produces evidence of His righteousness, power, purpose, and way in many forms. God has performed miracles, signs, and wonders in the sight of thousands of witnesses.
Without objective assurance from time to time, we would be living in a world of religious make-believe. God sometimes validates Himself before man by advertising His power through an undeniable occurrence like Jesus' resurrection (I Corinthians 15:1-8). Men have verified the truths of God through observation and experimentation (I Kings 18:30-39). Man is thus without excuse (Romans 1:18-25).
On occasion, God also verifies our personal relationship with Him by immediately answering a prayer or miraculously saving us from harm. On the other hand, if He needs to get our attention, He will shake us awake by allowing a test or trial to warn us that the relationship is degenerating. Because we are assured that God is with us, the testing is good. It keeps us from sinking into complacency and pride, both of which will separate us from Him.
This is what God is addressing in the principle of the plumb line. Amos understood that God was using it to test the spirituality, morality, and genuineness of the people against the standard. The test answers the question, "Are they really God's people?" God wants to know if they are exhibiting His characteristics.
This idea of a spiritual standard of measure transferred directly into the New Testament church. God uses similar imagery, a measuring rod, in Revelation 11:1. To the Laodicean church (Revelation 3:14-22), God uses fire to refer to a test instead of a plumb line.
As we can see from these examples, the end-time church will be tested. How are we going to build? What will the test reveal about our Christian growth (I Corinthians 3:9-16)? We are commanded to grow "to the measure of the stature of the fullness of Christ" (Ephesians 4:13). From this we see that the plumb line is God's revelation of Himself as the standard.
At first, God's revelation of Himself was direct, visible, and personal, but later, as Israel grew, He revealed Himself more verbally through the prophets. They recorded His revelation for all time and all people, and we read it today in our Bibles.
God's law is the primary vehicle He uses to reveal His nature; it defines how He lives. If we want to be in His Kingdom and live as He does, we must obey His law, but obeying God's law in no way minimizes grace. God revealed Himself to Israel first as Redeemer and then as Lawgiver. He freed His people from their slavery in Egypt before He gave them the standard of His law. Grace precedes law. God gives grace first, but He does not leave His people ignorant of the life that pleases Him, which is revealed in His law.
The plumb line combines grace and law, and God will test us against both. If we rely on His grace without law, or on His law without grace, we will not pass the test. If either is abused, we will not measure up to the standard.
Leviticus 19 shows that the revelation of the law is important because it is a verbal description of God's nature. Our God is a holy God (verse 2), and He expects His representatives to be holy also. But how do we become holy?
After God redeems us from sin and extends to us His Spirit and grace—His free, unmerited election, He expects us to follow His instructions. The remainder of Leviticus 19 fills in the details—we become holy by doing these things. These actions reflect God's nature. Since God is holy, His law is holy, and if we follow His holy law, we can—with the indwelling of His Holy Spirit—grow to be holy like our holy God.
God chose Israel and extended the offer for a relationship with Him, to walk and fellowship with Him. After Israel's rejection of it, He has now extended this offer to those He has specifically called and chosen (John 6:44; I Corinthians 1:26-29).
God loves His people and gives them redemption, grace. He expects it will result in obedience to His law, the reflection of His nature, so on occasion, He holds a plumb line against them to check their progress. But when He sees that they have rejected His way of life, He has no choice but to try to guide them to repentance—by any means necessary.