Romans 3:5

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.)

(v5, NIV)

After hearing what Paul has just said, the objectors make another objection. The objection calls Paul out on another contradiction. If God is just when he judges, then that means unrighteousness of men in some way leads to the glorification of God. Therefore, if Paul is calling out these Jews on unrighteousness and that they will be judged, they are also that which leads to the glorification of God. Therefore, the contradiction is that unrighteous people become that which glorifies God and you can't judge God glorifiers.

Here in this verse, Paul is simply stating this contradiction that he is being called out on. He ends this verse by saying that he is using a human argument, or speaking in a human way. He wants to make it clear that this kind of argument is the kind of argument that does not come from God. He wants it to be clear that this is not from God and it is not from him. This is something that he is using and he is simply re-stating this objection that he is hearing from his objectors. Just as a spoiler alert, Paul is about to defend himself again by finding flaws in their accusation. Paul isn't just going to sit there and take this. He won't stand for it and he brings about his counter-point in the next few verses.