I'm not sure I follow your objection, what is unclear in that pericope to you?
Given the rest of Paul's words in Romans 1-3, it seems not only reasonable to me, and pretty clearly so, that this section is setting up his discussion of the awareness of what God requires of us. That unbelief is not simply a matter of ignorance but a willful act of suppression. Objections regardinng the scope of this condemnation seem to me to be from modern views that see the question "does God exist?" as not only a coherent one, but pertinent to the issue. Perhaps you can clear up why you find the pericope unclear, because I see no room for ambiguity within it.
First of all, I'm not here to have a battle of wits, Fervent. I don't have time for that at the moment. But
I agree with you that Paul's epistemic statement "sets up" the spiritual and moral critique that he then lists out in reflection of a fallen, rebellious world of people. However, the short of it is that for us today, we can read his epistemic statement which the Romans would have likely understood and come away from it having little to no real idea as to its exact referents. All we can then make out today is the effects of that apparent epistemic denial to which Paul refers.
Therefore, what I'm getting at is that today, nearly 2,000 years later, we are now displaced in time and paradigm away from the necessary cultural and epistemic indicia that are needed to fully understand what Paul is (was) talking about. For us to even begin to try to justify some interpretation about his singular epistemic statement, we have to virtually go outside of the Scriptures in order to guess what "THOSE EPISTEMIC INDICIA" are which he says that all just somehow know by fiat from God. If we think we don't have to, then where in the Bible does it clearly say by "what" and "how" we are to know that God exist and what His moral evaluations are? Do we need to somehow study some later coagulation of thought that parses General Revelation from Specific Revelation, all of which takes a later systematic treatise to somehow tease out?
And if some folks today then try say that Paul was simply referring to the logical outlook that anyone at any time could discern in the makeup of the universe and in the makeup of our world and of humanity, but then actually mean by this that they think Paul was drawing in some bit of the Psalms or of Aristotelian logic (Prime Mover type assertion) to serve as his referents, then I think this is too circumstantial.
We don't know exactly what Paul thought and how and by what where epistemology is concerned. All we can strongly surmise about him and his thinking is that we was from Tarsus, a Pharisee, and a person with some connection to Gamaliel, and we can guess as to what his educational influences may have been which came to constitute his epistemic outlook.
In sum: we're guessing as to what Paul "means" in that specific verse, EVEN IF we don't really have to guess as to the meaning (mostly) in his later explication in the same chapter regarding the ravages of sin in ever increasing increments, all the way up to his stating that the worst thing for us to do is to not only sin without restraint, but to also approve of that very lack of self restraint, WE STILL THOUGH don't know on our own what specific referent is being alluded to that we're supposed to "somehow know" previous to all of this.
More can be said, but I'll stop there for now, with unsaid comments from the hermeneutical peanut gallery left to the side.