There are many avenues one may take to demonstrate the existence of God. As our starting point for this discussion, let's consider systems. There are all sorts of systems in the world: complex and non-complex-it does not matter what particular system we consider. But let us ask: "What is the most certain thing we can say about systems?" We can say, without a doubt, that a system is composed of simpler units. A system is a multiplicity of some kind, and so it is made up of parts. Moreover, a system depends upon the behavior of its parts. Emergent properties, for example, depend upon the interactions of the parts of the system (i.e., a swarm of bees, traffic, the market, etc.). Most systems are composed of parts which are in turn smaller systems, and these too are composed of parts or smaller units, which in turn are often systems unto themselves.
Now if the system, whatever system we are talking about, depends upon its smaller units, which may in turn be systems, we can determine with certainty that there cannot be an infinite number of smaller units upon which a larger system depends. How do we know this? We can employ the same reasoning (reductio ad absurdum) used to show that not everything can be "relative"--in the most general sense of that term.
What is relative depends upon something outside itself, in relation to which we understand it. For example, 'John is tall' is relative; for there is no "absolute" tall, only 'relative' tall. In other words, John is tall "in relation to" something other than John, namely the national average, or the class average, or the team average, etc. Without that "in relation to", it is impossible to come to a determinate or definite understanding of the claim: 'John is tall'. So, let's call a relative claim (like 'John is tall') the "final term" of a series. We both know John and we both agree that John is tall, because we both understand that in relation to which the claim is true, i.e., the national average (let's say John is 7',2"). The final term of a series will, if it is truly relative, depend on the term that is immediately prior, whatever that is. Let's label the final term Z, and its predecessor Y, and Y's predecessor X. In order to understand that Z is relative, I must at the same time know that Z is relative 'in relation to' Y. If I did not understand Z "in relation to" Y, if my understanding of Z did not depend upon anything outside itself, then Z would be understood "through itself" (per se), rather than 'in relation to' something other than Z, such as Y, and thus Z would not be relative. So, my understanding of Z depends upon my understanding of Y-if Z is truly relative. But we are testing the claim that everything is relative, so we have to maintain that even Y is relative, and thus my understanding of Y depends upon my understanding of X, whatever that turns out to be. Since everything is relative (or so we believe at this point), my understanding of X depends upon my understanding of W. So, in order to understand, here and now, Z, I must here and now understand Y, X, and W simultaneously-otherwise my understanding of Z is "indeterminate" (without "term" or end, that is, indefinite). Indefinite understanding, however, is unachieved. But I truly do understand that John is tall, and I understand that it is a relative claim (He's not absolutely tall, but only in relation to the societal average, or the team average, etc.). This means I understand, here and now, all the factors that are conditions for my understanding of the claim: "John is tall" (i.e., Y, X, W, etc.).
There cannot be an infinite series of "relatives" upon which my understanding of Z depends. If there were, I would never achieve a definitive understanding of Z, which is a claim that is relative. My understanding would depend upon an indeterminate (or infinite) number of factors, and so my understanding would be perpetually indeterminate, indefinite, without term or end. Hence, not everything is relative, and thus there is something that is "absolute". We need not know what that is. All we know for certain is that an infinite series of relatives is impossible.
Similarly, not everything can be a system. In other words, not everything can be a unit that is constituted by a multiplicity of smaller units-if the system depends upon those smaller units (as atoms depend upon subatomic particles, for example, or a society depends upon people, or a body depends upon cells, etc.); otherwise, the system that is constituted by an actually infinite number of smaller systems would never achieve the status of a determined system. Hence, there are units that are non-systems. These non-systems are one and indivisible.
This is what led the first atomists to say that the one indivisible unit (atomai = uncuttable) is being in its truest sense. The atomists claimed that the reality that we perceive outside of us is not being per se, but appearance, the result of the interactions of true beings or 'atoms'. We need not get into atomism at this point-for there are definite problems with it. But what is important is their insight that being is one and indivisible. They borrowed that from Parmenides. Here's how it works.
"Is" is one and indivisible. A circle, for example, is not indivisible-it can be divided into two (i.e., halves). By dividing the single quantity, which is the circle, it becomes multiple. Whatever has quantity is divisible, even if only logically divisible. But "is" or "being" in its most general sense has no parts. What is it that is outside of "is" or "being"? The answer is "non-is", or non-being, or what amounts to the same thing: nothing. In other words, what is "outside" of my hand? A possible answer is "my leg", for example. My leg is not my hand. This part is outside of that part, or "is not" that part. But "being" or "is" cannot be a "part". The reason is that outside of 'is' is non-is, or nothing. So "being" is one and indivisible. In sum, there is nothing outside of being. There is something outside of this or that circle, or this or that system, but there is nothing outside of "is" considered as such. Hence, being is not a quantity.
An infinite series of multiple units, in the here and now, that go to make up systems, which in turn make up larger systems, etc., is impossible; for no definite system would result. Multiplicity is eventually reduced to a single indivisible unit. What is that unit? This single unit is either at the bottom of the system, or at the top of the system. In other words, the unit determines the system to be, but it does so either from below and proceeds upwards or from the top and proceeds downwards. The single determines the multiple, which is to say that being determines the potential. The reductionist habit of mind tends to see the direction as proceeding from below and up towards the top. But that would seem to imply that the result, namely reality as it appears to us, is not being per se.
So, let's consider what it means to proceed from the top downwards. A being can be a system. For example, the human being is a complex system. But system describes "what" a thing is. Thus, system as system is not being, because being is one and undivided, and so the system must be determined by being. That by virtue of which a system is (or exists), cannot be a system. Hence, system does not explain being; rather, being explains system, at least ultimately. It is the act of existing (esse) of the system that accounts for the very existence of the system-as long as the system we are talking about is a single being-a swarm of bees or even a beehive, for example, is a complex system, but it is not a single being. "I", on the other hand, am a single being and I experience myself as such.
A multiplicity of beings, however, does require explanation, unless those beings contain within themselves the sufficient reason or explanation for their own being. As long as there is a distinction between "what" a thing is (system or not) and its very existence, that thing is contingent and does not contain within itself the sufficient reason for its existence. Whatever being contains the sufficient reason for its own being within itself will be "Being Itself" (its nature is "to be"), and thus absolutely One-there cannot be two beings that are "is" pure and simple.
At the very foundation of reality is a single, indivisible One. The relationship between this One and everything else is something for later, but at this point, let it be said that multiplicity cannot go on forever, just as "relativity" cannot constitute an infinite regress.
I am conscious of the fact that I know things outside of me (objects). When I close my eyes, I am also conscious of the fact that I am imagining or remembering things that are not me; these are objects of internal sensation. I know, but I also know that I know, or know that I am knowing things other than me. And so, my knowledge is twofold. I certainly know, albeit imperfectly, the object before me (objective knowledge), but I also know un-objectively, or subjectively, that is, I know myself as subject. I can certainly make myself the object of my knowledge, but in so doing, that objective knowledge of myself is at the same time accompanied by an intuition, a subjective knowledge, an awareness that I am knowing myself as object. This subjective knowledge, or knowledge of myself as "subject", is intuited and does not become objective. It is always behind me, so to speak.
There is much about myself of which I am aware. For example, I am aware that I am not necessary. What this means is that I know that I am an actualized potentiality-I did not always exist but do exist now. I also know that I am limited. Although I know myself as a being per se, thus relatively independent, it is also true that I am to a certain degree "relative"; for I know myself "in relation to" things other than me. And so, I know that I am not absolute. I have a profound sense of my own contingency-I am aware that I exist, but I am aware that existence is that which I "have", not that which I am-I cannot say that "I am being". I am a human kind of being, I am a complex being, a system if you will, but I am not my own existence. Rather, my existence is "had" or possessed-not possessed by a part of me, but by the whole of me. But that awareness of my own contingency (that I need not be) can only be had against the background of what is non-contingent, because contingency is a relative term, and as such can only be understood in relation to that which is non-contingent. In other words, on some level, I am aware of non-contingency, that is, I am aware of necessity, or that which "is" necessarily, and I know that I am not it. But that knowledge or awareness has not always been explicit; it has been implicit or preconscious before it was made explicit. That knowledge, which is an intuition, comes from the "subjective" plane or realm. It accompanies me always, but it is in the background, so to speak. It is a real knowledge that is non-objective, or subjective. In short, I know that I am not the Necessary Being, but the awareness of the Necessary Being is a condition that renders it possible for me to know myself as non-necessary or contingent.
This is the realm of spirit. Spirit includes a preconscious knowledge of God, which is not to be confused with an objective and explicit knowledge. Mystical knowledge, or an awareness of the presence of God (the Necessary Being), occurs in this realm, the realm of the superconscious. It is real, non-objective, subjective (not in the sense of a purely individual construct), spiritual, and superconscious.