...The testimony of the Lord is sure, making wise the simple.
In the first section of this psalm, David spoke on on how God's glory is revealed in what He has created. Starting in verse 7, he transitions to beholding the glory of God as revealed in His Word. It begins, "The Law of the Lord is perfect, converting the soul," disclosing its transformative power on the human soul.
In the second half of verse 7, David declares that the testimony of the Lord is sure, or reliable, faithful, or trustworthy. To learn of the gospel and its glorious promises, we need the written word. If God's word is not trustworthy and reliable, then our faith is in vain.
But David declares that the testimony of the Lord is unequivocally sure, and has the power to make wise the simple. From the practical wisdom revealed in the book of Proverbs to the "...Holy Scriptures, which are able to make you wise for salvation through faith which is in Christ Jesus" (2 Tim. 3:15), the Bible is like no other book on earth. In it is revealed the heart of God, the covenants of God, the plan of God, the kingdom of God, and the gospel of Jesus Christ.
Since the word of God can make one wise, we would do well to meditate on it day and night, as the Scripture says.