What does Psalm 39:8 mean?

A "transgression" is a breach of something; in this context it means violating God's law. In the Old Testament (Exodus 20), God clearly defined what His people were and were not supposed to do. A person could break God's law by failing to live according to His law. David admitted that he had broken God's law. Although he does not specify how he had acted contrary to God's law, he does make it clear that he longed for forgiveness. This might have been connected to David's sin with Bathsheba (2 Samuel 12:7–9).

In Scripture, the term "fool" can be used for someone unwise or irresponsible. It is frequently applied to someone who rejects godly wisdom (Proverbs 1:7). That use is a dominant theme of the book of Proverbs (Proverbs 10:8; 13:19; 18:7). David realizes that if he does not confess and repent of his sin, it would give "fools"—unbelievers and critics of the Lord—reason to criticize faith in God. It would also create room for those enemies to attack David's character.

Believers today should feel a sense of remorse over sins. Although we are not under the Old Testament law, as were the Israelites, we are accountable to obey the moral principles contained in the law since they reflect God's character. Those principles are inherent in the New Testament's precepts and commands as well. The apostle Paul told the Philippian believers to be "blameless and innocent, children of God without blemish in the midst of a crooked and twisted generation, among whom you shine as lights in the world" (Philippians 2:15).