“For what if some did not believe? Will their unbelief make the faithfulness of God without effect? Certainly not! Indeed, let God be true but every man a liar. As it is written: ‘That You may be justified in Your words and may overcome when You are judged.’” (Romans 3:3-4).
Even though God made and fulfilled an old covenant/testament and replaced it with a new covenant/testament, He is not fickle or unpredictable. His holy and righteous character remains perpetually perfect. Often in Scripture, an Old Testament passage is referenced to demonstrate the fact that a specific New Testament concept or doctrine is rooted in a righteous principle recorded in the Old Testament (see 1 Timothy 5:18, 19 for example).
God’s faithfulness is under consideration in the passage above. Paul’s quotation of Psalm 51:4 is intended to demonstrate the fact that God’s unfailing faithfulness is an everlasting truth about Jehovah.
Believing in God’s faithfulness to His word is not some fly-by-night, kooky idea cooked up in the overactive and fertile imagination of a tent maker from Tarsus. Certainly not! Rather, trusting God’s faithfulness to His holy character, His righteous principles, and His written word is what David did when he sought and received God’s forgiveness for his dastardly deeds involving Uriah and his wife.
In light of the unchanging, eternal principles of God’s unfailing faithfulness, a wise man realizes that human unbelief in God and His word does not negate any command, any promise, any warning, or any punishment He has spoken. Indeed, let God be true and believed tenaciously.
The Lord has spoken, and He keeps His word.