Romans 3:6

Certainly not! If that were so, how could God judge the world?

(v6, NIV)

Paul's plan is to show that his objectors have involved themselves in a contradiction. They are saying that you can't judge them because if God's judgment on them glorifies God, they become instruments leading to God's glorification. As a result, they are calling out Paul on a contradiction from what he has said. This may seem a bit confusing to follow the train of thought, but try to follow along. The objectors involve themselves in a contradiction. They say that you can't judge the unrighteous since they eventually lead to the glorification of God. However, if you can't judge the unrighteous, then how will God judge the world? These hypothetical objectors are probably the Jewish legalist hypocrites in chapter two. These would've wanted and very much desired for the judgment of the world. However, if they're calling out Paul on a contradiction, their objection is a contradiction because it wouldn't allow God to judge the world.