"Hearken unto me, ye that know righteousness, the people in whose heart is my law; fear ye not the reproach of men, neither be ye afraid of their revilings. For the moth shall eat them up like a garment, and the worm shall eat them like wool: but my righteousness shall be for ever, and my salvation from generation to generation." (Isaiah 51:7-8)
The Lord's words are purposefully pointed at those who love Him and want to follow Him. Undoubtedly, the God-fearing in Isaiah's day were mocked for their trust in God in the middle of international turmoil and instability. They were derided for their rejection of idolatry and likely persecuted for their love of God's Law (Isaiah 59:15). Yet, to the humble soul who sees the value of faith toward God and a life that demonstrates that faith, the Lord promises security and victory. To the regenerate, God's Law is not merely a compilation of letters. His Law has a heart behind the letters, and the letters are born out of God's Spirit. In the soul of God's redeemed is found the desire to keep God's Law because the saved man has changed authority. The words reflect the same heart behind the admonitions of John the Apostle. "For this is the love of God, that we keep his commandments: and his commandments are not grievous (I John 5:3)." David also spoke of God's Law being in the heart of the man who has accessed the righteousness of God through faith. "The mouth of the righteous speaketh wisdom, and his tongue talketh of judgment. The law of his God is in his heart; none of his steps shall slide (Psalm 37:30-31)."
The redeemed man is encouraged not to fear the temporary persecutions and rejections of the lost. Clothing and wool may be beautiful initially, but both are subject to decay. Both are extremely fragile when exposed to the right conditions; and both are temporal. Neither one can be preserved indefinitely. In contrast, the righteousness and salvation imparted by God are eternal. The righteousness and salvation mentioned are not found in man's keeping of the Law; but rather, they are found through faith in God's Messiah Who has flawlessly kept both the heart and letter of God's Law.