Of course, no discussion of the existence of God is complete without referring to St. Thomas Aquinas' "five proofs". His "five proofs" are the basis of many arguments from theists and atheists alike. Thomas wrote these almost 800 years ago, and so you'd think there was nothing new to say, but modern science has added some new observations. Let's look at them.
First Mover - since objects at rest remain at rest unless acted upon by an outside force (e.g. Newton's laws), the observation that objects are in motion implies an outside force to set them in motion. One can appeal to forces like gravity and explosions for things to "move by themselves" but each requires potential energy to have been put in the system (e.g. for a star to form via gravity it must have been formed from particles which were already separate against gravitational forces).
Philosophically speaking the "movement" can be something other than physical. I've couched this in terms of physics because it is the easiest for me to discuss. I never personally found this argument to be compelling because it seems to me that if the universe were always in motion, that motion could be simply transferred, as one billiard ball will set another in motion, and the second can come back and hit the first. It wasn't until college physics that I learned that this position doesn't work. More on this after I discuss the other two.
First Cause - things cannot cause themselves. Therefore, all that exists must have been caused to exist by something. To posit that the universe caused itself presents the dilemma refuted by Stephen Hawking in A Brief History of Time and Space, which he terms "turtles all the way down". Oddly, Hawking uses the argument that the universe caused itself in his latest book The Grand Design. I guess he only accepts logic when it supports his conclusions.
At first I found this argument unconvincing, because I had read books on quantum mechanics about how particles spontaneously came into existence in a vacuum. However, a vacuum isn't really nothing. As a physicist will tell you, there is space-time, and that is required for quantum laws to allow particles to come into existence. What makes space-time? According to Hawking gravity, a warping of space-time, creates space-time, which creates it. Beginning to sound like turtles?
The other argument I find used involves hypotheses like multiverses. I'll be covering these in a later post, but to whet your appetites, the scientific principle of Occam's Razor suggest that relying on something imagined to replace something you claim is imagined does not improve your position. Plus the multiverse is just another layer of "turtles". It does not address a first cause.
Contingency - all things in the observable universe are contingent on something else for their existence. Therefore the thing which brought them into existence existed without them. There must be something which exists outside of the universe.
Fr. Robert Barron, in his excellent PBS series "Catholicism" explains this in terms of a cloud on a Summer's day. The cloud exists because of the humidity, pressure and temperature of the air. Those conditions are caused by things like sunlight hitting the Earth, and the very composition of the atmosphere is influences by things like the respiration of plants. Those in turn depend on things like gravity, nuclear forces, evolution etc. Ultimately there comes a point where we get to the universe itself. To posit the universe is not contingent is not supported by any evidence, and in fact all evidence points to all things being contingent. Therefore it is reasonable to assume that there exists something outside the universe.
Of course, Fr. Barron is much more eloquent than I am. After all, he's a TV star. However, I think you get the point. And saying that the thing outside the universe is more universes (AKA the multiverse again) is falling back to the "turtle" argument.
The main thing about all three of these arguments is that they involve "the buck stops here". When people refer to these and say "turtles" they obviously haven't read or didn't understand these proofs. They are persuasive because to avoid them you have to postulate "turtles". You might ask "well, why can't the universe have gone on forever?" That was a position held by astronomer Fred Hoyle.
Fred Hoyle was a brilliant astronomer, cosmologist, and the man who coined the phrase "Big Bang" to disparage the theory proposed by Georges Lemaître. Monsignor Lemaître was a priest and professor of physics at the Catholic University of Louvain, who proposed that the universe was expanding from a single point, which he called the "primeval atom". Hoyle supported a theory called "steady state" which claimed that the universe was eternal and constantly renewed itself.
A detailed discussion of what went on would take more than this post, but suffice it to say Einstein's general theory of relativity coupled with Edwin Hubble's observational evidence, put the final nail in the coffin of the steady state model. Hoyle later recanted his steady state claims, and even left atheism and became a theist because of science. I'll cover his story in more detail in a future post as well, because his change of heart was due to science, not revelation.
That someone can today insist that the universe always existed, or regenerates itself simply does not fit with observed scientific evidence or with established laws of physics. For instance, the second law of thermodynamics states that entropy never decreases. Any claim that the universe exists forever or regenerates has to assume a magical way that this and other laws of physics can be suspended. I see no reason to think that this sort of imaginary process is any more likely than God.
The last two proofs of Thomas are different.
Greatest Being - some things are greater than other things. Therefore, there must be a being of perfect greatness, from which all other things derive their greatness.
This argument is based on ancient Greek philosophy, but I find it unconvincing, since there is an assumption that such a perfect being exists as proof that one exists. One argument in favor is that existence is greater than non-existence, so if the greatest being did not exist, it would not be the greatest being (and therefore the greatest being must exist).
I don't hold with the assumption that if we can imagine a being it must exist. Ironically, that is an argument used by some atheists to support the "all possible timelines" model of explaining the universe. They assume that the reason the universe exists is because everything that can exist must exist. By that argument, of course, God must exist (unless He can't exist, which is something they will argue without evidence).
Intelligent Designer - purpose requires intent. Since there are things in the universe that work towards a purpose, they must have been an intelligence who directed them towards that purpose.
This is another argument I find weak, because it supposes that the universe has a purpose without offering evidence to that end. I believe there is evidence to that end, which I will explore in a future post, but without that evidence, it would seem to be a circular argument - it relies upon intelligence behind the universe to prove intelligence behind the universe.
Your mileage may vary. I know some find one proof more compelling than another. However, I have yet to see a refutation of the first three proofs that does not rely upon a logical or scientific contradiction, or just plain wishing. If you know of one, I would be more than happy to hear it.
There are dozens more proofs to be covered and lots of counterarguments to debunk, so this series is for now open ended. The next post will begin to cover a different class of proofs.