Then what advantage has the Jew? Or what is the value of circumcision? Much in every way. To begin with, the Jews were entrusted with the oracles of God. 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.”

But if our unrighteousness serves to show the righteousness of God, what shall we say? That God is unrighteous to inflict wrath on us? (I speak in a human way.) By no means! For then how could God judge the world? But if through my lie God’s truth abounds to his glory, why am I still being condemned as a sinner? And why not do evil that good may come?—as some people slanderously charge us with saying. Their condemnation is just.

The issue here is not simply that God gave a good command and it resulted in sin.  “Why not do evil that good may come?”  God entrusted Israel to do a task and promised to bring about a result through Israel.  But he proved faithful in bringing about this good result through Israel’s unfaithfulness.

Israel broke covenant with God and thereby were instrumental in bringing about the New Covenant that was promised.  No wonder people would mock Paul’s Gospel, saying, “Why not do evil that good may come?”