Sea Breeze: A materialistic view of the nature of man is the underlying problem that you and others are having.
Not exactly. Recognizing the physical nature of the universe is not a presupposition, nor does it require one. It reflects the most basic understanding of the world that surrounds us.
A person may decide to take up the presupposition that the material world is all that there is, but this is an unnecessary complication, since it requires that he demonstrate a negative. It is perfectly reasonable to accept that the material universe exists and is measurable, and to expect evidence for any claims that posit anything outside of the physical.
By definition, a presupposition is made and kept in light of the lack of evidence. If you have evidence, then you do not need to presuppose an idea. Religious presuppositions are easy enough to spot, since they are supported by attempts to fit them into gaps in our knowledge or understanding. There is no evidence for gods or souls (or similar spiritual concepts) that doesn't take the form of "if X is not true, then how do you explain Y?" Try it. Find an approach that doesn't amount to one of the following:
If A is not true, then how do you explain B?
If A is not true, then how do you account for C?
How does D make sense, if A is not true?
How is E possible without A?
...and so on.
This should come as no surprise. If there was a way to measure such things and come up with consistent outcomes, the world would follow one religion and one god (or however many gods there might be). Lacking this evidence, mankind has worked its way through dozens --if not hundreds-- of religions and thousands --and possibly hundreds of thousands-- of denominations within those religions. Without evidence --without something we can test-- presuppositions are all we have. And they are not enough.
As an example, there is no need to presuppose that water can exist in three different states. We discovered this through observation and confirmed how it works through research and testing. We know most (perhaps all) of the combinations of temperature, air pressure, and other factors that determine the forms that water can take. The experiments are both obvious and simple.
Not only is there no test for determining the triune nature of Yahweh, but there is no consensus on the matter among people who share almost all of the same presuppositions related to the god of the Bible. There is no way to test or confirm the 'triality of man,' either. Theists themselves will admit that this cannot be done; they depend on the Bible and on the centuries of constant interpretation of its words and meanings. And --as we can see in some of the discussions here-- we seem no closer to figuring it out than we were centuries ago.
The natures of water and deities are not comparable. The metaphysical does not have the same legitimacy as the physical. The latter is undeniable. The former is as yet only a concept. Without actual evidence, that will not change.