An illustrational form tells you through the intelligence immediately what the form is about, whereas a non-illustrational form works first upon sensation and then slowly leaks back into the fact.
On the subject of the nature of the gods, the first question is Do the gods exist or do the not? It is difficult you may say to deny that they exist. I would agree if we were arguing the matter in a public assembly, but in a private discussion of this kind, it is perfectly easy to do so.