“The law of the Lord is perfect, reviving the soul; the testimony of the Lord is sure, making wise the simple;” -Psalm 19:7
Though David’s immediate motivation is to remind Israel of their particular relationship with God, we who are under the new covenant (Christ) are ever yet benefited by the Law of God. We are not antinomians. That is, while we are aware of Paul’s analogy in Romans six reminding us that we are under grace and not the law, we are also aware that he means sin no longer has reign over us because we have died (the wages of sin under the law) in Christ. That is what our baptism signifies. We are now to give our members (mind and bodies) to be servants of Christ rather than servants of sin (Romans 6:1-19). In Christ, we have passed from one state of being (dead in sin under the Law) to another state of being (Alive in Christ under grace).
But Christ did not come to abolish the Law. He came to fulfill it (Matthew 5:17). Thus what David says to Israel about the Law is still true for us today. The Law furnishes us, as it did for Israel, a “more certain and intimate knowledge” of God’s majesty.
The Law here is the Ten Commandments (or Statements, in Jewish Theology), and more. It encompasses the Commandments as well as the Covenant, the ceremonies, and the edicts and ordinances. The Law (Torah) of the Lord is blameless, without blemish. God’s law is not tyrannical or unjust. It is the most perfect expression of righteousness, thereby safeguarding human dignity and flourishing. Rightly oriented worship is foundation for human flourishing; thus, it revives the soul of those who obey it.
Secondly, the testimony (Eduth) means “witness,” “decree,” or “confirmation.” It is most often understood as relating the Covenant God made with Israel, a renewal and expansion of that Covenant made with Abraham. He set before them life and death, blessing and cursing (Deuteronomy 30:15-21). It is trustworthy to give wisdom to mankind, especially those who are young, naive, immature, or inexperienced.