“What if some were unfaithful? Does their faithlessness nullify the faithfulness of God? By no means! Let God be true though every one were a liar, as it is written, “That you may be justified in your words, and prevail when you are judged.”” -Romans 3:3–4
Paul anticipates the likely response of his imagined interlocutor and uses the answer to set up the next confirmatio: God alone is righteous and faithful.
The question is, since you’ve now proved the Jews—privileged as they were to receive the oracles of God—were unfaithful, does that mean their faithlessness has nullified the faithfulness of God?
Paul’s answer is that the faithfulness of God is so far from being hindered “by the perfidy and apostasy of men,” that its opposite is actually established.
Quoting from Psalm 116:11 and Psalm 51:4, Paul establishes that while all men are inclined to falsehood and deceit—yea, they always shun the truth and run after lies—on the contrary, God will always stand by his promises because whatever God declares he fulfills, despite the falseness of mankind. Whatever God commands will become a reality no matter what men do.