The context: Romans 3:1-5 (NIV) “What advantage, then, is there in being a Jew, or what value is there in circumcision? Much in every way! First of all, the Jews have been entrusted with the very words of God. What if some were unfaithful? Will their unfaithfulness nullify God’s faithfulness? Not at all! Let God be true, and every human being a liar. As it is written: ‘So that you may be proved right when you speak and prevail when you judge.’ But if our unrighteousness brings out God’s righteousness more clearly, what shall we say? That God is unjust in bringing his wrath on us? (I am using a human argument.)”
The Apostle Paul is here nullifying what could be a ridiculous human argument that somehow our unfaithfulness might be esteemed as God being unfaithful to us or the Jewish people. Whereas quite the contrary is true; God is faithful to us even when we are unfaithful to him. Paul then cites a scripture written by David to prove this thought in Psalm 51:4 (NIV) “Against you, you only, have I sinned and done what is evil in your sight; so you are right in your verdict and justified when you judge.” The Psalms scripture is probably a little easier to understand.