I'm sure we've all heard people claiming that their holy scriptures are true without the merest hint of rationale. Well, mine is true by default.
Let's put aside the worries about theistic justification as I don't think you need this claim anyway. But this is what I'm seeing come out of this and why I'm having trouble engaging.
This Immutable Omnexicon contains all and only true information. Now to be clear, I can conceive of what this content would be like even I don't know what the content is. But herein lies the problem and the distinction to be drawn between this and 'traditional religion'. The question comes down to how we should think about the content of the Omnexicon.
Based on the idea that you are approach the content definitionally, then we might have something very close to the following:
NT: This sentence is necessarily true.
or alternatively:
NT*: The content of this sentence is necessarily true.
For a number of reasons, I don't think this is the way you want to go. But the reason that's crucial for your purposes is that neither NT nor NT* carry any weight with respect to... well... anything! In short, your argument won't be able to get off the ground with this understanding of the content of the Omnexicon.
The better approach, then, is to give some story about epistemic access. You could say, "Look, the Omnexicon contains all and only true information. We obviously don't have epistemic access to all truths, so we don't have access to the content of this book. But rest assured, the content is there and it does the work that I say it does."
This is at least conceivable and something I can get on board with. Plus, the Omnexicon can do some work for you under this conception of its content.
But here's the downside. The theist would claim that we *do* have epistemic access to the truths contained in their holy texts. This allows their texts to do a number of things:
1) They provide us with certain constraints on what we ought to believe (this is actually a dicey claim, but it's in the ballpark).
2) They are ethically normative - that is, they tell us how we ought to act or what kind of person we ought to be.
3) They provide us access to (alleged) truths that aren't obviously contained in the world around us.
It's pretty clear, however, that the Omnexicon can't accomplish any of these things. By having only a conceptual understanding of its content without proper epistemic access, we can't really say what - if anything - the Omnexicon can get for us. Does it oppose the evangelisation of other deities? It's not part of my conception of the truth contained therein, so I would be dubious about that claim. So it looks like we've ended up in the same boat as the previous option.