[Update Jan 29, 2018: This gets uncomfortably close to theological speculation. I’m fumbling toward a point that I think can be made in a valid way, but this may be the wrong approach. Take it with a grain of salt.]
I promised something big. Let’s see if I can deliver.
God is perfect, so a perfect understanding of him should have perfect consequences. By the same token, improved understanding should always help and a correct understanding should have better results than an incorrect one. Therefore, provided we can identify what consequences a proper understanding of God should have, and how those consequences would come about, we may be able to reason about the relative merits of different understandings. Now, the fun step:
Beliefs held by temporal beings have material consequences in the temporal world.
Given God’s perfection, these material consequences should also be good – and they are observable. Furthermore, given that the Revelation is complete, these consequences should accrue with minimal divine intervention, according to the ordinary workings of the temporal world.
Therefore, to the extent we understand divine purpose, we should be able to reason about proper theological belief based on the ordinary consequences of holding it in the temporal world.
Now, we may never fully understand divine purpose, but we may be able to discern parts of it. To the extent we are, we may reason about the attributes of God that would serve the purpose. For example, knowing God should help us know the truth.
I’ll be working on that next.
[My understanding of the problem keeps evolving as I make progress. I shifted my focus again. That’s what I get for mentioning my plans.]