During a debate between Christian apologist William Lane Craig and physicist Sean Carroll (watch the video), Carroll enumerated several points in support of his view that theism was a failed hypothesis. As the first of these, he complained about a lack of evidence. If theism were true, Carroll said, there's no reason for God to hide. Carroll indicated, with apparent sincerity, an openness to consider evidence. Because the debate took place in a chapel, he invited the ceiling to fall in on him. He looked up. He even seemed to cringe slightly during a brief moment of uncertainty. Nothing happened. He shrugged. No evidence.
One of the main reasons naturalists like Carroll have so much trouble finding evidence of spiritual reality is that they look in the wrong places. Here's a simple analogy: I claim I have a microwave oven in my home. If you come over and look in the bedrooms, the living room, the hallway, every closet, and even the yard, but refuse to go in the kitchen, you won't find any evidence to support my claim. To find my microwave oven, you have to look in my kitchen.
Naturalists insist on considering only physical evidence. By its very definition, spiritual is not physical, so it is unsurprising that—to naturalists looking only at material things—it seems to be hiding. Spiritual reality, if it can be said to be hiding at all, is hiding in plain sight. Spiritual evidence is ubiquitous and so obvious many people simply look past it. Ignoring it altogether, they don't see what is right in front of them. Or if they do see, they disregard it.
Some of the nonphysical, spiritual things that permeate human society include interior dialogs and a host of everyday emotions such as love, joy, and anxiety. Others involve the ability to make sense of abstraction and metaphor, such as discovering something true in art, finding meaning in written words, or following the logic of a mathematical proof. Spiritual reality is also involved in planning, hoping, and forgiving.
Even people who deny the existence of a spiritual realm regularly participate in these activities. They often claim that these things are merely physical functions of the brain. This notion is partly correct. Modern neuroscience has indeed identified certain types of brain activity and chemical processes associated with spiritual experiences. This is exactly the same way in which visual, auditory, or tactile experiences produce specific brain responses. When naturalists evaluate these brain responses, they claim the ones produced by physical stimuli result from those stimuli. But, when they evaluate the brain's responses to spiritual stimuli, they say there's no evidence of a cause. Well, not a physical one anyway.
Beyond these mundane examples of the spiritual realm at work, countless individual witnesses around the world and throughout recorded history have testified to life-changing encounters with something other than the physical. During the debate's question and answer period, no one asked Carroll what he would say to these people who have had spiritual experiences equally as dramatic and convincing as a chapel ceiling falling on command. Nevertheless, history records the actions of many people who have made astonishing claims. Furthermore, modern-day statistics indicate that 84% of the world's current population asserts a religious affiliation (The Global Religious Landscape). These 5.8 billion people routinely devote themselves to tasks such as seeking wisdom in sacred writings, using prayer as a means of communicating with a transcendent entity, and dedicating time and resources toward reaching spiritual goals.
When five out of every six people on the planet claim some level of connection with a spiritual reality, it seems problematic to assert that any deity or other mystical representation of transcendent reality is in hiding. In fact, plenty of people have found something and many have dedicated their lives to help humanity better understand it.
Different hypotheses have been proposed. Various experiments have been conducted, and robust research continues. The fact that the evidence is so overwhelmingly abundant possibly contributes to Carroll's next objection: that if spiritual reality were true, all religious people would report identical interpretations of the evidence. I'll discuss this concern next.