As David continues to expound on the perfections of the Word of God in the latter half of Psalm 19, he argues for its truthfulness and reliability. David writes, “The testimony of the LORD is sure, making wise the simple” (v. 7). The Hebrew word for sure speaks of truth that God Himself testifies of or gives witness too. The language also describes not only that which is firm or secure, but that which has been verified. The same Hebrew word is used that way in Genesis 42:20 where Joseph says, “Bring your youngest brother to me, so your words may be verified.” In other words, the Word of God is verified in its truthfulness, dependability and reliability.
Charles Spurgeon writes, “God’s witness in his Word is so sure that we may draw solid comfort from it both for time and eternity, and so sure that no attacks made upon it, however fierce or subtle, can ever weaken its force.”
David goes on to give the believer a direct application of the reliability of God’s Word. “The testimony of the LORD is reliable, making wise the simple” (v. 7). The Hebrew word for simple describes someone who is wide open in terms of easily falling prey to every conceivable influence, and to whatever this life has to offer. If you read the book of Proverbs you’ll find the simple person described as the person who simply doesn’t have enough life experience to understand how things work. Proverbs 14:15 says, “The naive believes everything, but the sensible man considers his steps.”
Not only does Scripture provide wisdom for salvation, it is also able to give you the wisdom necessary for living in this world in a way that pleases God. Do you believe that Scripture is God’s verified testimony to you and that if you spend time in it, you will no longer be a simple person? Are you convinced that Scripture is non-negotiable for someone who desires to live wisely for the glory of God?
John Calvin writes, “When we give ourselves up to be guided and governed by the word of God, we are in no danger of going astray, since this is the path by which he securely guides his own people to salvation.”
 Treasury of David
 Commentary on the Psalms