We can’t prove God’s existence in the way that one can “prove” a theorem of mathematics, like the Pythagorean theorem or the existence of infinitely many prime numbers – by pure logic from axioms everyone accepts.
God and Natural Reason
However, we can know by natural reason (and not only by faith) that there is a God. This knowledge is available to anyone with fully functioning rational faculties. However, these faculties can be damaged by sin (both individual and societal).
To know God’s existence, we must be willing to follow reason to its utmost limit.
Doubt can be Healthy or Unhealthy
A healthy doubt enables us to think more clearly and responsibly, by protecting us from irrational fads, myths and fables.
A healthy doubt promotes a prudent humility and openness to new information.
However, doubt, when carried to extremes, actually paralyzes thought, making us doubt those first principles without which all knowledge would be impossible.
The Error of the “Enlightenment”
The “Enlightenment”, beginning with the French philosopher and mathematician René Descartes (1596-1650) and the British philosopher John Locke (1632-1704), created the Myth of the Method.
According to this Myth, all human reasoning can be reduced to a mechanical recipe, a set of procedures that all can follow to reach the right conclusions, without the need of exercising judgment or wisdom.
Post-Modernism: The Overreaction
The Enlightenment idea experienced a series of catastrophic refutations in the 19th and 20th centuries, as philosophical rationalism failed to produce the universal consensus that the Enlightenment had promised.
In modern times, post-modernism has arisen as a natural reaction to this failure: denying the very existence of human reason or of a universal Truth (with a capital T), as opposed to “my truth” or “our truth”.
The Post-Modern Mindset
Post-modernists are suspicious of any claim to have or to know any universal truth.
Quantum Mechanics teaches that some events are uncaused
No -- it teaches that some events are caused but not pre-determined in every respect.
God cannot exist necessarily.
What we can imagine existing, we can imagine not existing.
What we can imagine is possible.
So, if God’s existence is possible, so is His non-existence.
Not everything we can imagine is really possible (time travel, infinite regresses).
We cannot imagine God’s existing or not existing: God’s being is beyond our comprehension. However, we can know that He exists (by the First Cause argument).
The first cause argument supposes that God’s existence is necessary.
To be absolutely necessary, God would have to exist by definition.
However, nothing can exist by definition. Suppose, for example, that I defined a ‘schmountain’ as a golden mountain that exists.
We cannot conclude that schmountains exist by definition, since that would entail that gold mountains exist.
We are not saying that God exists by definition – just that He is uncausable by definition. Golden mountains are not uncausable. We conclude that God exists by argument, not by definition.
If such a God exists, He exists by virtue of His own nature. Only an infinite and simple being could exist in this way.
Why believe the principle of causality?
Perhaps it [the principle of causality] just expresses an arbitrary demand; it may be intellectually satisfying to believe that there is, objectively, an explanation for everything together, even if we can only guess what the explanation might be. But we have no right to assume that the universe will comply with our intellectual preferences.
J. L. Mackie, The Miracle of Theism: Arguments for and against the Existence of God (Clarendon Press, Oxford, 1982), p. 86.
Is the Argument Self-Contradictory?
...the causal argument is not merely valid but self-contradictory: the conclusion, which says that something (God) does not have a cause, contradicts the premise, which says that everything does have a cause.
John Hospers, An Introduction to Philosophical Analysis, 2nd edition (Prentice-Hall, Englewood Cliffs, NJ, 1967), p. 431.
Objection: Why Can’t Matter be Ultimate?
Aquinas’s implicit assumption is that anything whose essence does not involve existence must, even if it is permanent, depend for its existence on something else... we have no reason for accepting this implicit assumption. Why, for example, might not there be a permanent stock of matter whose essence did not involve existence but which did not derive its existence from anything else?
J. L. Mackie, The Miracle of Theism: Arguments for and Against the Existence of God (Clarendon Press, Oxford, 1982), p. 91.
Couldn’t the Universe itself be a First Cause?
No – the universe is merely an aggregate of things, each of which is variable and contingent.
What about the Big Bang?
We know that it is finite in many dimensions (the fine-tuned constants, entropy, velocity).
It isn’t ultimate: it isn’t obviously uncausable. God is – by definition.
Objection: Why stop with God?
All three of these arguments (Aquinas’s first three ways) rely upon the idea of a regress and invoke God to terminate it. They make the entirely unwarranted assumption that God himself is immune to the regress. Even if we allow the dubious luxury of arbitrarily conjuring up a terminator to an infinite regress and giving it a name, simply because we need one, there is absolutely no reason to endow that terminator with any of the properties normally ascribed to God: omnipotence, omniscience, goodness, creativity of design... Richard Dawkins, The God Delusion, Houghton Mifflin, Boston, 2006, p. 77
But the greatest weakness of this otherwise attractive argument is that some reason is required for making God the one exception to the supposed need for something else to depend on: why should God, rather than anything else, be taken as the only satisfactory termination of the regress? If we do not simply accept this as sheer mystery..., we shall have to defend it in something like the ways that the metaphysicians have suggested. J. L. Mackie, The Miracle of Theism: Arguments for and Against the Existence of God, 1982, p. 92.
Can God be Ultimate (Self-Explanatory)?
It is utterly impossible for God to be self-explanatory, Theists usually consider God's existence to be part of His essence and maintain that this renders God self-explanatory. But what do we mean by a certain characteristic’s being part of the essence of a thing? The essence of a thing is what it is. The essence of a table, for example, is whatever is common to all particular instances of tables. Having a flat surface on top is a characteristic which is part of the essence of a table. But the fact that anything that is a table must have a flat surface does not explain why this particular thing was constructed as a table -- that is, why it is a table. It merely explains why the thing must have a flat surface if it is a table. Similarly the fact that God has an essence which includes existence would not explain why God is a being which has this particular essence. It only explains why he must exist if he has the particular essence he happens to have.
B. C. Johnson, The Atheist Debater’s Handbook (Prometheus Books, NY, 1983), p. 66.
Is God utterly Unknowable?
Despite the complicated maneuvers of Christian theologians to escape from agnosticism, despite the abstract dissertations and impressive sounding attributes of God, it always comes down to this: the nature of God is “entirely unknown” to man. The characteristic under consideration -- the identity of God's essence and existence -- is nothing more than an extremely complicated way of conceding the very point which we are attempting to prove: that the very concept of God is without cognitive content. The Christian cannot give substance and meaning to the term “God”; it is a blank, an unknown “something”.
George H. Smith, Atheism: The Case Against God (Prometheus Press, NY, 1989), p. 66.