Romans 3:4b

Not at all! Let God be true, and every man a liar. As it is written:

"So that you may be proved right when you speak

and prevail when you judge."

(v4, NIV)

Here, Paul is defending his argument by deflecting an objection about the faithfulness of God. What Paul had said previously seems to bring about the conclusion that God is no longer faithful according to these objectors. However, God does not cease to be faithful if he judges the Jews. God is true even if every man is a liar. In order to back up his point, Paul quotes Psalms 51:4.

Against you, you only, have I sinned

and done what is evil in your sight,

so that you are proved right when you speak

and justified when you judge.

(Psalms 51:4, NIV)

In this Psalm, David is crying out to God about his transgression with Bathsheba. David himself lines up with what Paul is saying. God is justified when he judges, even if he is to judge David and even if he is to judge the Jews. We sin and God is still faithful and just. He judges us and is proved right when he speaks and justified when he judges.