What is the thing you use the term "God" to describe?
You may answer that the thing is the Creator of the Universe, Definer of Human Purpose and Meaning, Source of Morality, All Powerful Divine, Supreme Spiritual Being, etc. However, these are just other names for "God." They are personified summations of the roles "God" may fill and characteristics "God" may embody. In other words, such answers convey the who of "God," but not the what.1
After ruling out who answers, you may hesitantly offer suggestions like "love" or "energy." But, in all likelihood, these descriptions fall short of even beginning to clearly explain the mysterious thing you are using the term "God" to describe. Plus, love is love and energy is energy! For many people, the what of "God" can only be confidently defined as something like "a substance that simply cannot be adequately or accurately described using words."
The term "substance" here is important. Most people do not conceive of "God" as a physical thing made of material, but as a metaphysical thing. Metaphysical things are said to be composed of materials undetectable to humans, but they must be comprised of some kind of substance otherwise they simply wouldn't independently exist. So "substance," in this case, refers to a type of immaterial material.2
If you agree with everything that has been said thus far and do not see the problem already posed by the what of "God," it is likely because you are automatically filtering the word "God" through a framework using a process I call Re-Concretion.
Re-Concretion entails the abstraction of a noun into adjectives and adjectival phrases and subsequent linguistic solidification of the abstract descriptors back into the original noun.3
If someone processes a term for an abstract concept using Re-Concretion, it usually means he does not fully comprehend that concept. This unawareness frequently manifests itself in ungrammatical usages of the term. For example, if you were to show the new iPhone to a member of an indigenous tribe completely disconnected from modern civilization and say something a little tongue-in-cheek like, “This is brought to you by technology,” he will likely conceptualize “technology” as some sort of mystical power. All he has to go off of is that whatever “technology” is somehow created your strange light device and imbued you with the power of long range telekinesis, so he will probably personify the word in ways you would deem ungrammatical. For instance, he may mention how he wants to “meet Technology.”
However, when it comes to metaphysical things like “God,” filtering words through the Re-Concretion process is standard procedure for many people. When “God” believers think of what the thing is they use the word “God” to describe, they are likely thinking about personifications of abstract who answers provided by a religious framework. Theistic religions anthropomorphize the roles they claim “God” fills and characteristics they claim “God” embodies, so, when asked about the what of “God,” followers of such religions likely conceptualize anthropomorphized roles and characteristics rather than a substance that simply cannot be adequately or accurately described using words.
This conflation of the personified who with the what metamorphizes "God" from a metaphysical thing comprised of immaterial material to an abstract thing composed of adjectives and adjectival phrases. Not only does this present a stark definitional contradiction, it poses a strange conundrum for the faithful: Do they believe in the thing "God" actually is or in a man-made construct contrived to interpret something supposedly uninterpretable?4
The foremost issue with the Re-Concretion of "God" is that the abstract conceptions (the who) do not mesh with the metaphysical understanding (the what). The claim that "God" is a substance that simply cannot be adequately or accurately described using words cannot be reconciled with the claim that "God" possesses X specific set of characteristics. One must be able to know the what before he can know the who.
Put otherwise, one cannot possibly know what roles "God" fills or characteristics"God" embodies if he doesn't even know what "God" is. If one truly believes he cannot know the what of "God," he must concede that all conjectures and speculations about the who of "God" are equally valid (or invalid) for he has absolutely no grounds on which to claim another's descriptions are inaccurate if he doesn't even know what is being described.5
However, the what of “God” proves problematic even if one is operating with a “God" conception uninfluenced by Re-Concretion. “God” as immaterial material cannot possibly exist for it is a logical contradiction; it has just as much chance of being real as a married bachelor.
In all honesty, the opening question was loaded. Any answer one feels adequately describes the what of “God” will either be something with a more specific and less mystical name like “love” or “energy” that cannot possibly contain the characteristics attributed to the who of “God” or a logical contradiction. However, despite – or maybe to spite – these fundamental problems, many will continue to believe their conception of “God” exists outside their imagination. A large percentage of these people do not recognize their continuous conflation of the personified who with the what resulting from their filtration of the term “God” through the Re-Concretion process and perhaps that’s for the better, but those who value intellectual honesty and rationality may wish to investigate what exactly it is they are calling “God” lest they enslave themselves to unfounded personifications or logical impossibilities.
Notes
1 One heuristic for testing whether an answer addresses the question "Who is X-Thing?" rather than "What is X-Thing?" is to capitalize it and replace it with the word denoting "X-Thing" in the former question. If the same question is still being asked, then the answer functions as a synonym, as another name for X-thing. It is likely a who answer rather than a what answer.
For example, say a friend who had never seen a dog before asked you, "What is the thing you use the term 'dog' to describe?" Would you respond with answers like Man's Best Friend, Most Loyal of Pets, Occasional Destroyer of Couches, Chief Exporter of Slobber, Purest Source of Unalloyed Joy, or Barker of Barks? Do these answers effectively convey the what of "dog" or do they merely rephrase your friend's question? Clearly the latter! Such answers may do the job for someone who already knows what a dog is and just forgot the word "dog," but phrases like the ones above would certainly fail to elucidate what a dog actually is to someone who does not already possess a clear conception.
A much better response to your friend's question "What is the thing you use the term 'dog' to describe?" would be "a domesticated carnivorous mammal that typically has a long snout, an acute sense of smell, non-retractable claws, and a barking, howling, or whining voice" (the Oxford Dictionary's definition of "dog"). This answer provides your friend with a mental picture of a dog and the fundamental understanding that a dog is an animal composed of biological tissue.
2One may argue that immaterial things like physics, feelings, and thoughts face similar definitional challenges as "God." This is a thoughtful objection, but it conflates two fundamentally different things: the abstract and the metaphysical.
If all of humanity was to die, abstract things would cease to exist whereas metaphysical things would survive. Abstract things like physics, feelings, and thoughts are composed of human perception and/or reason -- they are forwarded as immaterial conceptions, but not immaterial material.
Metaphysical things like "God," however, are not forwarded as immaterial conceptions that only exist in the minds of living beings, but as living things themselves. Metaphysical things like "God" are regarded as compositions of immaterial material that would continue to exist even if all of humanity was to die.
(Of course, the physical phenomena the theories of physics are used to interpret would still exist if all of humanity was to die, but physics itself as a field of study would not. Where would physics as a field of study exist if all humanity was to die? If it doesn't exist anywhere, it cannot exist at all.)
3Re-Concretion is akin to the common practice of adjectivization which most typically involves converting a noun into an adjective by adding a suffix, i.e., "father" adjectivized is "fatherly," "business" adjectivized is "businesslike," "harm" adjectivized is "harmful." The important distinction between normal adjectivization and Re-Concretion is that words undergoing Re-Concretion do not change form and are thus often definitionally confounded. Re-Concretion can also seem similar to metaphorization, but the words produced by Re-Concretion are not forwarded as comparative metaphors, but exact references.
4Author Peter Rollins refers to such constructs "conceptual idols" in his book How (Not) to Speak of God. I do not share Rollins's perspective, but I admire his candor regarding the unintelligibility of "God" and am indebted to him for his linguistic analysis of the term.
5Some may object claiming it is possible to possess knowledge about something while simply lacking the vocabulary necessary to explicate that knowledge using words, but I encourage such objectors to keep in mind the difference between something that is difficult to describe and something that is impossible to describe. If something is difficult to describe, it can still be described. But if something is impossible to describe, it simply doesn't exist. What existing things are impossible to describe? If you think of something, I'd love to hear from you.