Psalm 19:7: Law and Testimony
The law of ʏʜᴡʜ is perfect, reviving the soul. The testimony of ʏʜᴡʜ is trustworthy, making wise the naive person. (Psalm 19:7 [Heb. verse 8] my translation)
In the first six verses of the psalm (seven verses in the Hebrew numbering) much is made of the heavens, including the Sun, as messengers of God's glory, craftmanship, and power. The wonders of the sky are an exciting encouragement to the faith of the believer, and an overwhelming challenge to the unbelief of the skeptic. A feature of this message is that it is available to all people in all places, even the most remote and isolated tribes of mankind.
Yet there are limitations to this natural revelation. It is God's special revelation, communicated in words, which is needed to change our inner person and to provide crucial spiritual insight. Though heaven and earth pass away, the word of God endures, and it should be more valuable to us than all earthly treasures and delights.
The law of ʏʜᴡʜ is perfect, reviving the soul.
The word "perfect" here is the Hebrew word תְּמִימָה (thᵉmị-mā), which in its various senses refers to that which is "complete", "without fault", and "impeccable". The word is used often for animal sacrifices that lack any fault or blemish. Deuteronomy 32:4 is a helpful cross-reference, speaking of God's perfection:
The Rock, his work is perfect (תָּמִים, thā-mịm) for all his ways are justice. A God of faithfulness and without iniquity, just and upright is he. (ESV)
The Hebrew word מְשִׁיבַת (mᵉshị-vot) means to bring something back, and so the phrase מְשִׁ֣יבַת נָ֑פֶשׁ (mᵉshị-vot na-fesh) has traditionally been translated "converting the soul". However, when these words are used together, they should be understood as "reviving" or "revitalizing" the soul, that is, bringing life back to the inner person.
The testimony of ʏʜᴡʜ is trustworthy, making wise the naive person.
The word "testimony", translating עֵדוּת (ʿē-dūt), is obviously used here as a synonym for God's word, but its fuller meaning is likely to be unclear to the modern reader. To the Hebrew listener, it would have brought to mind the "testimony" or "witness" of what God had done on behalf of his people the Israelites — including redeeming them out of slavery in Egypt and bringing them into the promised land — and the obligations they had to God and to each other because of that.
He established a testimony (עֵדוּת) in Jacob and appointed a law in Israel, which he commanded our fathers to teach to their children, that the next generation might know them, the children yet unborn, and arise and tell them to their children, so that they should set their hope in God and not forget the works of God, but keep his commandments. (Psalm 78:5-7)
In a similar way, Christians have a testimony of how God has rescued us from the power of sin and from condemnation, and has made us new creations in Christ. Concomitant with this is the responsibility to live our lives according to the principles of the gospel, pursuing holiness and love, and bearing the fruits of the Spirit. This is reflected in the ceremony of water baptism, in which we testify before the world the death, burial, and resurrection of Christ, our own transformation in Christ, and our formal commitment to walk in newness of life.
Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death? We were buried therefore with him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life. (Romans 6:3-4, ESV)
I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service (Romans 12:1, KJV).
According to our passage, the testimony of ʏʜᴡʜ is trustworthy, making wise the naive person. The word "trustworthy" translates נֶאֱמָנָה (ne-ʾămā-nā) which can also be translated "faithful". The word more generally refers to that which is firm, reliable, and enduring. The "testimony" is trustworthy, and it does not fail of its promises or leave us disappointed. The simple or ignorant person who chooses to know and to follow it becomes wise in learning to follow the good and rewarding path.
 See HALOT.
 For example, Exodus 12:5.
 Compare with Ruth 4:14-15 and Lamentations 1:11.
 For further helpful study, look up the references to the "testimony" and the "tablets of the testimony" throughout the book of Exodus, specifying the "stipulations of the Sinai covenant laid down in writing". The use of the word here in Psalm 19 is more general and not necessarily referring to a specific document or tablet (see HALOT) but is certainly tied back to the covenant and the testimony focused on in the book of Exodus. The Davidic kings also had a "testimony" given to them at their coronation, evidently scripture laying out their royal obligations to YHWH as related to the Davidic covenant: see 2 Kings 11:12 and 2 Chronicles 23:11.
 See HALOT.