Here I will give my responses to arguments for the existence of God. I will try to include all of the notable ones that I've heard throughout the years. I continue to have friendly debates with theists (mostly Christians) and will add more arguments if I come across new ones.
The Argument from Design
This is a common one. Everything is so complex and apparently designed for us. Everything from physical constants to the eye couldn't have just happened. If you find a watch in a desert, do you assume that it formed itself or that someone designed it? If a tornado indefinitely passes over parts to a Boeing 747, a fully constructed jet will never be the result. Obviously an all-powerful, all-knowing, timeless, spaceless, loving, merciful, just, personal god is behind everything.
Where to begin? Starting with what I think should be a basic path to truth, a claim is as justifiable as the evidence that supports it. "I don't know, therefore God" is not a sound logical argument. In order to propose God as a cause, one must be able to demonstrate that God both exists and is in fact the cause of a proposed effect. If there's no known physical explanation for something, that doesn't mean that there's necessarily a supernatural one. Plus, the supernatural realm as a whole is a massive claim that has yet to be proven to even exist.
Even if there were no natural laws that could result in the perceived complexity and design of the universe, God wouldn't be an intellectually honest conclusion. Rather, "I don't know," is all one can say with no evidence to go on. Yet, as it turns out, there are natural explanations for complex life and even the elements that make life possible.
As for the watch, how does one contrast between what forms naturally and what was designed? A watch is not part of nature and can't reproduce, so it was surely designed; but life forms like trees and humans are part of nature and reproduce. Not only is there no evidence of a creator, but one isn't needed because evolution beautifully explains how life evolved and continues to evolve.
Evolution and the Big Bang Are Just Theories
Evolution is a word that seems to be feared or laughed at by many Christians. Pastors tend to mock and belittle evolution and much of science along with it. "It's just a theory," they say. Yet, so are atomic theory, molecular theory, cell theory, germ theory, the theory of plate tectonics, heliocentric theory, the theory of relativity, and many others. Do these same people refuse to wash their hands before eating because germs are just a theory? I doubt it.
A scientific theory is different from a layperson's use of the word. In common, everyday speech, if I have a theory for why some people don't tip at restaurants, it's a hunch. It's likely an uneducated guess that's as good as the next person's idea. In science, however, a theory is much different. To quote Wikipedia,
"A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of knowledge that has been repeatedly confirmed through observation and experiment. Scientists create scientific theories from hypotheses that have been corroborated through the scientific method, then gather evidence to test their accuracy. As with all forms of scientific knowledge, scientific theories are inductive in nature and do not make apodictic propositions; instead, they aim for predictive and explanatory force.Evolution is both a fact and a theory. DNA evidence alone proves it beyond a doubt, but the fossil record helps scientists understand when and where life forms went extinct and how they lived; and vestigial organs such as wisdom teeth and the appendix show how body parts may have been used in previous forms. It's been observed in the lab and in nature; notable examples include the bananas we eat, the many breeds of dogs that humanity has recently given rise to, and new strains of bacteria that are resistant to antibiotics.
The strength of a scientific theory is related to the diversity of phenomena it can explain, which is measured by its ability to make falsifiable predictions with respect to those phenomena. Theories are improved as more evidence is gathered, so that accuracy in prediction improves over time. Scientists use theories as a foundation to gain further scientific knowledge, as well as to accomplish goals such as inventing technology or curing disease.
Scientific theories are the most reliable, rigorous, and comprehensive form of scientific knowledge. This is significantly different from the word 'theory' in common usage, which implies that something is unsubstantiated or speculative."
Evolution is the most unifying theory in all of biology, yet it's very simple to understand for those that don't refuse to learn about it. However, some creationists accept microevolution but deny macroevolution. Biologists don't distinguish between the two, and here's why: they're the same thing. The only difference is time. Some people expect dogs to evolve wings or some drastic change to happen within their lifetime; aside from the fact that dogs evolving wings wouldn't make sense, evolution happens at the genetic level. The best way to find results is to geographically separate a quickly-reproducing species into environments with different selective pressures. Fruit flies are a famous example of a life form whose evolution was observable because of their short life spans. Yet, even at their rate of reproduction, within a human lifetime they'll still look like fruit flies. Some people expect what took hundreds of thousands of years to happen to take place within their lifetimes. Plus, "it's still a fly" is subjective in the sense that "fly" is a name that people give to a wide variety of species. Chihuahuas and great danes are both considered dogs, but they're quite different from one another.
As for the Big Bang, the expanding universe is universally accepted in astronomy and confirmed by the redshift of other galaxies. If you rewind the expansion backwards, the energy and matter in the universe were once in a condensed state. There's also the cosmic microwave background, which is thought to be leftover radiation from the Big Bang.
Lastly, I want to discuss the ages of the Earth and the universe, something that intertwines with both evolution and the Big Bang. Multiple fields of science using various methods independently confirm that the Earth is roughly 4.5 billion years old and the universe 13.8 billion. The only people that I know of that push the idea of a young Earth that's 6,000 - 10,000 years old are young-Earth creationists that seem to be using biblical events to calculate the Earth's age. Consider that we can see objects as far away as 13.2 billion light-years, which means the light left those objects 13.2 billion years ago. YECs say that God created the light already here (and supposedly the Grand Canyon's layers that would've taken much longer to form, the fossil record's lack of out-of-place fossils, samples from space that just so happen to be aged at around 4.5 billion years using over a dozen methods, etc.), but last August the light from a supernova from the galaxy M101 — which is 21 million light-years away — reached the Earth. Did God bring the light all the way toward us but stop only a few thousand light-years away so that it would reach us then?
The point I want to make is that evolution and the Big Bang are strongly-supported scientific theories. By being called theories, they have to have been rigorously tested and match observations. They must be falsifiable. All it would take to throw a wrench in the theory evolution would be a single out-of-place fossil! Yet, the closest to that I've seen is a disproven set of claims that some combination of a human footprint or fossil was found next to a dinosaur footprint or fossil. Evolution in particular is a beautiful theory that explains the relationship that all life forms on Earth have, whereas "God did it" hasn't been proven, nor can it be.
There's Great Debate in the Scientific Community
While most scientists do accept evolution, it's worth noting that not all of them do. Similarly, not all historians believe that a historical Jesus existed. There's a Darwin dissent list where several scientists signed saying that they "are skeptical of claims for the ability of random mutation and natural selection to account for the complexity of life." The numbers may seem large at face value; but remember that the total number of scientists in the world is much larger. To again quote Wikipedia,
"There is a notable difference between the opinion of scientists and that of the general public in the United States. A 2009 poll by Pew Research Center found that 'Nearly all scientists (97%) say humans and other living things have evolved over time – 87% say evolution is due to natural processes, such as natural selection. The dominant position among scientists – that living things have evolved due to natural processes – is shared by only about third (32%) of the public.'"Interestingly, some Christians accept evolution and say that God used evolution to create complex life forms. The problem with those that don't seems to be that if evolution is true, then a literal interpretation of Genesis becomes more difficult. If humans evolved, then there were no "first humans" because evolution is a gradual process with minor changes from generation to generation; if Adam and Eve never existed, then what should readers make of original sin?
There Has to Be a First Cause
It's not known whether or not all of existence had a beginning, although I'm confident to say that the total energy and matter is finite (even if it's zero). I say existence instead of this universe because there could be other universes, although I'm not claiming that there are. Since there's no pre-Big Bang data to go by, any ideas of what it was like are just ideas. We could be in the only universe, and it may or may not have begun to exist. There could be other universes; some scientists suggest that universes might be like bubbles that pop into and out of existence. I heard a profound idea while listening to a lecture by evolutionary biologist Richard Dawkins, which was that if universes have the ability to create other universes, such as by fission, and if physical constants can differ from universe to universe, then maybe there's a natural selection of universes where only the most "fit" universes survive long enough to form new universes, with these also happening to be the ones suitable for life.
It's all speculation that far back in time, if it even makes sense to talk about time then (Lawrence Krauss suggests that it doesn't since time didn't exist). However, if the universe did begin to exist, does that necessitate a god? As with abiogenesis, some things in science aren't yet known and may never be. When there's no evidence, "I don't know" is the most intellectually honest stance to take. There could be a finite number of universes whose total energy and mass never began to exist, although this is also speculation. Since this is a possibility, I don't see God as a necessity. I also don't rule out the possibility of gods completely, but as a skeptic and an admirer of Carl Sagan's famous line, I demand extraordinary evidence when presented with extraordinary claims.
Furthermore, the idea of a god raises even bigger questions than it answers, such as how that god has the power it does and can live outside of space and time. There's the problem of infinite regress, which theists respond to by saying that God doesn't need a cause; he just is. Unfortunately, it's special pleading. If God always existed and doesn't require a cause, then the universe could have always existed without requiring a cause. If god is infinitely greater than the universe, then his existence demands a greater explanation.
You Really Believe in God; You Just Want to Sin
Some theists claim that there are no atheists, that atheists know that God is real and choose to reject him because they don't like having to live by certain rules. I can't speak for all atheists, but I would actually prefer a reality in which there was some form of an afterlife. The idea of non-existence after death isn't very appealing. I'm not an atheist because it brings me comfort but because I care whether or not what I believe is true.
In fact, according to Protestantism, the only way to sin and get away with it is to be a Christian and accept Jesus as savior. For those that believe in "once saved, always saved," if you accept Jesus, you can sin all you want and go to heaven. Sure, if they're really saved, they won't want to sin, but my point is that wanting to sin isn't motivation to profess disbelief in God. If atheists did believe in God, they'd know they couldn't outsmart him by pretending not to believe.
As an ex-Christian, I'll even say that part of me misses being a Christian. There were times that I slipped away, but it was always worth it to get right with God. I was filled with joy when I knew for certain that I'd one day be in heaven with him and as I watched other people get saved. It brought me comfort to believe that the creator of the universe cared about me and loved me so much that he sent his only son to die in a painful and humiliating way just so that my sins could be washed away. To someone that believes in God, everything happens for a reason, and death is only a temporary separation from loved ones before a reunion in paradise.
Yet, even if I wanted to believe in God again, I couldn't just choose to. You couldn't make yourself truly believe that the Sun is a green ice cube because it contradicts what you know. You could pretend to and try really hard to think that it is, but deep down you'd know that it's actually a giant sphere of plasma. I would need to be convinced. The quickest way for this to happen would be for God to appear and say, "Here I am."
If God is omniscient, omnipotent, and benevolent, he knows what it would take to convince each individual of his existence and has the power and the will to do so. Yet, the vast majority of people aren't Christians, let alone any specific kind of Christian. Some claim that revealing himself would violate free will, but that's simply not true. I believe that the tree outside exists, but I'm not forced to worship it. If the violation of free will would come from not being able to disbelieve in God, then the Bible itself disproves this, since it claims that none are without excuse and that God's existence is proven through creation and the conscience. There's also the fact that people don't choose what they believe, as evident by the average's person's inability to fully believe that the Sun is a green ice cube.
The Bible Is the Proof You're Asking For
When there are no other arguments to make, the Bible is thrown at people (figuratively, of course). I constantly hear that the Bible has 66 books, written by 40 people, on three continents, in three languages, over a 1600-year period. There are no contradictions except for a few scribal errors. It's inspired of God, or God-breathed. Many prophecies have come true including 9/11, and more are to come. It's the most sold and revered book in the world.
What you won't be as likely to hear at church is that there are no original manuscripts. This tells me that in the period of hundreds of years of copying and translating, prophecies could have been written in, and inaccuracies could have been corrected. Also, why weren't even the original documents in the New Testament written until decades after Jesus died? If he was so important and truly performed miracles, more people would've written it down, and some of those earlier copies should've survived as come claim that the current Bible did thanks to God preserving it.
Have you ever played Telephone? It's a game where a message is spread from person to person. The last person inevitably says something radically different from what the first person said. You may also be aware of the big fish phenomenon as I like to call it. Person X catches a fish, and every time the story gets told the fish grows in size. Eventually what was a tiny fish becomes a giant sea monster. The stories eventually written in the Bible were told orally from person to person, who then told other people and so on. Because of the gap in time between the actual events and when they were written down, much could have been exaggerated or fabricated. There are also no definite eyewitness accounts of Jesus, at least among canonical works.
Here's an even more important question: why would the creator of the universe rely on a man-made, man-copied, man-translated, man-interpreted book written in languages that would die out to spread his word? Why would this book be so ambiguous that it would spawn over 33,000 denominations that disagree with other from trivial details to the fundamentals of the religion, such as how to get into heaven, whether or not a literal hell exists, whether or not women can be pastors or teach, how much alcohol if any is okay to drink, and how literally to interpret Genesis?
Ironically, I have started studying the Bible and plan to continue. When I was a Christian I read most of the New Testament and paid attention in church during sermons that covered a variety of books of the Bible, but I want to study it more and maybe even learn Hebrew and Greek to study the earliest manuscripts available. Why, if I don't believe in God? It will improve my debating skills and give me a better understanding of a book that has affected the lives of many people throughout history. I don't have to agree with Hitler or intend to follow his path if I read Mein Kampf. It will be neat to use an understanding of the Bible when debating Christians.
To be continued.