“Twice in this brief section of Isaiah’s prophecy, the words ‘law’ and ‘testimony’ are used together. Elsewhere, the law delivered to Moses is called ‘the Testimony.’ This is an important concept. God's law is the testimony of His perfect character. As a testimony, it functions both positively and negatively.

From the negative perspective, the law is a testimony against our sin, exposing the fact that our thoughts, words and deeds do not conform to God's perfect righteousness. From the positive perspective, the law is a testimony of God's holiness. Sin corrupts the image of God in man, and the law bears testimony to this dark reality.

When God regenerates a soul, His image is restored in the sinner. The converted sinner’s conformity to God's law then testifies to the glory of God, who alone can accomplish such a work of grace. When our worship does not conform ‘to the law and to the testimony,’ a different message is sent. False worship is a testimony to the sufficiency of man, and is for this reason called ‘will- worship.’

Calvin writing on this passage from Isaiah, says: ‘Hence we learn that everything which is added to the word must be condemned and rejected. It is the will of the Lord that we shall depend wholly on his word, and that our knowledge shall be confined within its limits; and therefore, if we lend our ears to others, we take a liberty which he has forbidden, and offer to him a gross insult. Everything that is introduced by men on their own authority will be nothing else than a corruption of the word; and consequently, if we wish to obey God, we must reject all other instructors. He likewise warns us that, if we abide by the law of God. we shall be protected against superstitions and wicked modes of worship.’

The rallying cry of those who seek God's glory in worship is this: ‘To the law and to the testimony! if they do not speak according to this word, it is because there is no light in them.’” (Comin, 219-220)