Campbell is working under the assumption that Romans 1-4 serves as the textual base for justification theory. That's a reasonable assumption. It's the only place in the Bible that has such a sustained discussion of justification. You would then expect to see the key components corroborated in this text. The thing is, we have several underdeterminations; cases where the text doesn't actually ever say what justification theory requires.
One key component of justification theoy is the 'perfectionist axiom.' This is the claim that God requires perfect righteousness in order to be justified without exception. Interestingly, Romans 1-4 never makes that claim. The issue seems to be a thoroughgoing depravity not the failure to be perfect. One then recognizes that this exacerbates the problems we ran into earlier. How can any so thoroughly depraved person grasp their own depravity and evaluate themselves and their need for a savior without outside help (which would be an epistemologically different system from justification theory)? (347-9).
In addition to the underdeterminations, we have a lot of material that's unneccessary to support justification theory. One that's particularly thorny recurs in Romans 2. Who are the righteous saved? Since no one is righteous by works (according to the perfection assumption), why create them as a category? It's even weird for them to function as a hypothetical, since it's a category that's impossible for anyone to fall into. Justification theory can't explain why they appear at this point (367-8).
Again, these are just two of the 35 exegetical problems that Campbell enumerates. These combined with some of the problems we discussed in earlier posts in this series make justification theory problematic enough that we need to start looking for alternatives.