What does Psalm 40:10 mean?

As in the prior statement (Psalm 40:9), David affirms that he did not keep secret what the Lord had done for him. He told the great congregation—the worshipers in the sanctuary—about the Lord's reliable rescue. Divine attributes such as trustworthiness and love are expressed by God towards all of His people.

Amidst lamenting what befell Jerusalem, Jeremiah consoled himself with the assurance that the Lord is faithful and loving at all times. He wrote: "The steadfast love of the LORD never ceases; his mercies never come to an end; they are new every morning; great is your faithfulness" (Lamentations 3:22–23). The salvation David alludes to here is likely the deliverance the Lord gave him when David was hunted and attacked by his enemies in the wilderness.

Following David's example, we too should not keep hidden the truth that the Lord is always reliable and loving. Nor should we keep it a secret that the Lord has saved us. He has delivered us from our sins and eternal judgment, and what He did for us He can do for others. But, as Romans 10:14 asks: "How are they to believe in him of whom they have never heard? and how are they to hear without someone preaching [proclaiming]?"