I'd be curious as to the rationalization of a spirtual NT to be honest. A type characterized by the rational worldview they impose upon the world seems, on the surface, at odds with belief in a being that doesn't make itself present empirically. When I was younger, I certainly "hung on" to the thought of a deity as I began distancing myself from an upbringing that included relgion but ultimately I simply had to cast it aside if I was being honest with myself.
"The rational worldview they impose upon the world"? This suggests that the world is not itself inherently rational. Much that can be observed empirically is rational, though, and follows principles that we can understand and use to predict future events. There is also the realm of chaos, events that at least so far have not yielded to such analysis. I do not understand chaos theory well enough myself to know the extent of its useful predictive ability, or whether it is mostly human wishful thinking.
Though we will never prove the existence (or nonexistence) of God, that doesn't mean there is no evidence. At least my personal spirituality is informed by experiences and observations, not just fears and imaginings. Granted, this evidence is pointing to something non-corporeal, but I accept other intangibles on limited evidence as well, usually because it is practical and useful to do so. Even established scientific theories are never really proven. We just accept them and use them until they are shown to be false or we find data they cannot account for. We think there was a big bang, and that species evolve through natural selection, not because we witnessed the actual events, but because these concepts explain the phenomena we are able to witness directly. Scientists continue to refine both theories as we learn more.
I do not mean by this to suggest that some day, we will be able to provide a material, physical explanation of everything attributed by believers to "God". Over the course of human history, we certainly have replaced many supernatural explanations with natural ones, and we probably will continue to do so. I believe, though, that there will always be more to the universe, and to existence, than the material aspects that can be explained through scientific analysis. But this, too, is a matter of belief.