In its content the law is also a portrait of God, hence, the testimony of God (Psa. 78:5; 119:88b). The law, the Ten Commandments, is a photo of God. Because it is a photo of God, it is a testimony. This is like saying that our picture is a testimony of ourselves. The psalmists referred to the law as the testimony of God.
The function of the law is to expose men in their sinful nature and evil deeds (Rom. 3:20b; 4:15b; 5:20b; 7:7b). In this sense, the law works as a mirror. If we do not look in a mirror, we cannot see how dirty our face is. But when we look at the mirror, the dirty condition of our face is exposed. The law works like a mirror to expose our sinful nature and evil deeds.
Furthermore, in its function the law was used by God to guard His chosen people and escort them to Christ (Gal. 3:23-24). Paul told us in Galatians that before Christ came, God gave His chosen people the law as a guardian to guard them. Then when Christ came, this guardian was used by God to escort and bring them to Christ. By being exposed by the law, subdued by the law, and convinced by the law, God's chosen people would go through the law to Christ.
The law also functions to subdue men as sinners under God's judgment (Rom. 3:19). We need to see the law in this light. God's purpose in giving the law was to expose man, to subdue man, and to guard or keep God's chosen ones for Christ that they may be led to Christ. If we stayed only in the book of Psalms, we could not receive this kind of light. This light comes altogether from Paul's teaching.