For what if some [Jews] 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)
After showing that the Jew will stand before the Judge for their sin the same as the Gentile and that the Jews did have an advantage - they knew the Scriptures - Paul turns his attention toward some of the Jew’s objections.
The force of the objection is this: “If we suppose that the Jews were unfaithful, as you claim Paul, then it is to be supposed that the Jews would be lost. This would in turn imply that God had failed to keep His promises.” - Wacaster, Studies in Romans, 106
Does the Jew’s unfaithfulness nullify the faithfulness of God?
What is it that some of the Jews did not believe?
I think Paul had in mind the promises of God concerning the Messiah. Throughout the Old Testament, God had given promises of the Messiah - and when Jesus arrived on the scene, many of the Jews rejected Him. They were expected to respond in faith to the Messiah.
Without developing it yet, Paul has inferred by “for what if some were without faith?” that faith is bound upon them. Later in the book he will make the case for faith as a condition of blessing. See especially 4:13-16. - McGuiggan, Romans, 112
But when some Jews failed to respond in faith to God’s promised Messiah - did that mean that God was unfaithful to His people and His promises?
Wasn’t the proclamation of the gospel of Jesus Christ and the salvation found in Him evidence that God had not been faithful in His promises to the Jews?
Paul strongly disagreed with that conclusion.
Paul recoiled in horror at the thought of God being unfaithful. His challenge was to convince the Judaizing element in the Roman church that the God of his Law-free gospel was faithful to His covenant promises and to His people. - Pollard, Romans, 107
Paul quoted Psalm 51:4 to showcase the trustworthiness and faithfulness of God to judge according to righteousness. David had sinned with Bathsheba, and in Psalm 51, he acknowledged his sinfulness and God’s righteous justice.
Just as God was not unjust to bring justice upon David and his house for his sin, so God would not be unfaithful to the Jews by judging those who rejected the promised Messiah.
These two verses are a powerful reminder that God always keeps His promises. Even if every person was a liar - God is still faithful and trustworthy.
Should all men be proved to be false, it shall never be so with God. He will forever stand faithful to His promises. What great encouragement to those who have put their trust in God, to know that He will keep His promises, without the failure of even a SINGLE ONE! - Wacaster, Studies in Romans, 106