How to Argue That God Does Not Exist
To argue that God doesn't exist, try to focus on the scientific and cultural evidence. Point out evidence of evolution that predates the theory of creationism. For example, scientists have used radiocarbon dating to prove that there are fossils over a millions of years old. While there's a lot that science can't explain, you could argue that the gap of what we don't know is getting smaller, and it's only a matter of time before people won't need religion to explain things. You could also argue that believing in God is not necessarily a natural human trait. For example, people who aren't exposed to religion or the idea of God as kids don't tend to believe in God as adults. There are also many contradictions within religious texts. You could argue that these demonstrate a flawed model for God's existence.
Propose that living beings are poorly designed. The argument from poor design states that if God is perfect, why did he create us and many other living creatures so poorly? For instance, we are vulnerable to many diseases, our bones break easily, and with age our bodies and minds break down. You could also mention our poorly-made spines, inflexible knees, and pelvic bones that make childbirth difficult and painful for women.  Together, this biological evidence indicates that God does not exist (or that he did not create us well, in which case, there is no reason to worship him). Believers might counter this argument by stating that if God is perfect, then he created us as well as could possibly be expected. They might also argue that what we see as imperfections actually have a purpose in the larger workings of God's design. Point out the logical fallacy in this right away. We can't live our lives hoping that one day an explanation for why our eyes or shoulders were designed so poorly will arise. Reference the philosopher Voltaire, who wrote a novel about people looking for meaning after a devastating earthquake hit Paris. We are pattern-seeking animals, so naturally we look and hope for patterns where none can be found. Some might point out that God originally created humans in their perfect form, but after humanity sinned against God, God's original creation became corrupt and wrought with sin, and death and entropy entered the world as a result. Be aware of this rebuttal when using the flawed-design argument. Show the history of replacing supernatural with natural explanations. The “God of the Gaps” argument is common when people argue that God exists. It argues that whiles modern science can explain many things, it cannot explain others. You can refute this by saying that the things we do not understand are decreasing every year, and that while natural explanations have replaced theistic explanations, supernatural or theistic explanations never replace scientific ones. For instance, you might cite the example of evolution as one area where science has revised previous God-centric explanations for the variety of species in our world. Argue that religion has often been used to explain the unexplainable. The Greeks used Poseidon to explain how earthquakes happen, which we now know is due to the movement of tectonic plates to relieve pressure. Demonstrate the inaccuracy of creationism. If the existence of the world can be explained purely in scientific terms, then it is unnecessary to say that God brought the world into being. According to the principle of Occam's razor , the simplest explanation is generally the best. Creationism is the belief that God created the world, usually within a relatively recent time-frame like 5,000-6,000 years ago. Draw on the wealth of reasonable evidence that disproves this, such as evolutionary data, fossils, radiocarbon dating, and ice cores to argue that creationism is false and that belief in God is unnecessary. For instance, you might say, “We find rocks all the time that are dated to be millions or even billions of years old. Doesn't this conflict with the belief that the universe was recently created by God?” Some might argue that the earth only appears to be old because Noah's Flood dramatically changed the climate and geology of the earth. However, this fails to explain the millions of craters on the moon and the supernovas in outer space. Argue that belief in God is socially determined. There are several variations on this idea. You could explain that in relatively poor nations, almost everyone believes in God, while in relatively rich and developed nations, few people believe in God. You could also state that people who are well-educated are more likely to be atheists than those who have lower levels of education. Together, these facts make a strong case that God is only a cultural product and belief in God depends on one's particular social circumstances. You could also suggest that people who are raised in one religion overwhelmingly tend to stick with that religion throughout their lives. Those who were not raised in a religious household, conversely, rarely become religious later. Explain that just because most people believe in God, it is not necessarily true. One common reason for belief in God is that most people believe it. This “common consent” argument might also suppose that because belief in God is so high, such a belief must be natural. However, you can refute this idea by proposing that just because many people believe something, it is not right. For instance, you might explain that belief in the Greek gods was common but is now no longer socially acceptable. Suggest that if people are not exposed to religion or the idea of God, they will tend to not believe in God. Explore the variety of religious belief. The identities and characteristics of the Christian, Hindu, and Buddhist gods are very different. Therefore, you could argue that even if God exists, there is no way to know which God should be worshipped. This is known formally as the argument from inconsistent revelations. Demonstrate the contradictions within religious texts. Most religions offer their holy texts as both a product of and evidence for their God. If you can demonstrate that a holy text is inconsistent or otherwise flawed, you'd provide a solid justification for God's nonexistence. For instance, if God is described in one part of the holy text as forgiving, but later wipes out a whole village or country, you could use this apparent contradiction to demonstrate that God could not exist (or that the holy text is lying). In the case of the Bible, often entire verses, stories, and anecdotes were falsified or changed at some point. For instance, Mark 9:29 and John 7:53 to 8:11 contain passages that were copied from other sources. Explain that this demonstrates that holy texts are just a mishmash of creative ideas produced by people, not divinely inspired books. Argue that if God exists, he would not allow so much unbelief. This argument proposes that where atheism exists, God would descend or intervene personally in the world to reveal himself to atheists. The fact that so many atheists exist, however, and that God has not attempted to persuade them through divine intervention, means that God likely does not exist. Believers might counter this claim by stating that God allows free will, and that therefore, unbelief is an inevitable outcome of this. They might cite specific instances in their holy texts of occasions when their God revealed himself to people who still refused to believe. Explore inconsistency in the other person's belief. If the believer's faith is premised on the idea that God created the universe because “All things have a beginning and an end,” you could ask, “If that is so, then what created God?" This will emphasize to the other person that they are unfairly concluding that God exists when in fact, the same basic premise (that all things have a beginning) can lead to two different conclusions. People who believe in God might counter that God -- being omnipotent -- is outside of space and time, and is therefore the exception to the rule that all things have a beginning and an end. If they counter in this way, you should direct the argument towards the contradictions in the idea of omnipotence. Explore the problem of evil. The problem of evil asks how God can exist if evil exists. In other words, if God exists and is good, he should eliminate all evil. "If God truly cared about us,” you could argue, “there would be no wars." Your conversation partner might reply "Governments by man are ungodly and fallible. People, not God, cause evil.” In this way, your conversation partner might again invoke the idea of free will to counter the notion that God is responsible for all the wickedness in the world. Still, this counter fails to explain the evil that is not caused by humans, such as sickness from microorganisms and earthquakes. You could also go one step further and argue that even if a bad god exists who does allow evil, he is not worth worshiping. Demonstrate that morality does not require any religious belief. Many people believe that without religion, the planet would descend into immoral chaos. However, you can explain that your own behavior (or that of any other atheist) is little different than that of the believer. Admit that while you are not perfect, nobody is, and belief in God does not drive people to necessarily be more moral or righteous than anyone else. You could also reverse this proposition by arguing that not only does religion not lead to goodness, it leads to evil, since many religious people commit immoral acts in the name of their God. You might, for instance, draw attention to the Spanish Inquisition or religious terrorism around the world. In addition, animals who are incapable of understanding our human concept of religion show clear evidence of instinctual understanding of moral behavior and distinguishing between right and wrong. You could argue that morality is a social behavior that helps ensure the collective survival of a species and is not necessarily spiritually linked. Demonstrate that a good life does not require God. Many people believe that only with God can one live a rich, happy, and full life.  However, you could point out that many people who do not believe are happier and more successful than those who are religious. For instance, you might draw attention to Richard Dawkins or Christopher Hitchens as individuals who met with great success despite the fact they do not believe in God. Explain the contradiction between omniscience and free will. Omniscience, the ability to know everything, seems to be at odds in most religious dogma. Free will refers to the idea that you are in charge of your actions and therefore responsible for them. Most religions believe in both concepts, but they are incompatible. Say to your conversation partner, “If God knows everything that has happened and will happen, as well as every thought your mind creates before you think it, your future is a foregone conclusion. That being the case, how can God judge us for what we do?” People who believe in God may answer that although God knows an individual's decision beforehand, individual actions are still each person's free choice. This idea is a nice thought, but it is still contradictory for the reasons above. Show the impossibility of omnipotence. Omnipotence is the ability to do anything. If God can do anything, though, he should be able to, for instance, draw a square circle. However, since this is logically incoherent, it makes no sense to believe that God is omnipotent. Another logically impossible thing you could suggest God cannot do is to know and not know something at the same time. You could also argue that if God is omnipotent, why does he allow natural disasters, massacres, and wars? Some believers offer the idea that perhaps God isn't completely omnipotent, and that while he is extremely powerful, he cannot do absolutely everything. This may explain why God can do some things but cannot logically do others. Put the ball in their court. In reality, it is impossible to prove that something does not exist. Anything could exist, but in order for a belief to be valid and worthy of attention, it needs hard evidence to back it up. Propose that rather than arguing that God does not exist, the believer needs to provide evidence that God does exist. For instance, you could ask what happens after death. Many people who believe in God also believe in an afterlife. Ask for evidence of this afterlife. Spiritual entities such as gods, devils, heaven, hell, angels, demons, and so on have never been (and cannot be) scientifically examined or observed. Point out that these spiritual features cannot be proven to exist if they are not observable and measurable. Do your homework. Prepare to argue that God doesn't exist by familiarizing yourself with the main arguments and ideas of well-known atheists. God is Not Great , by Christopher Hitchens, for instance, is a good place to start. The God Delusion , by Richard Dawkins, is another excellent source of rational arguments against the existence of a religious deity. In addition to researching arguments in favor of atheism, investigate the rebuttals or justifications from the religious perspective. Be familiar with the issues or beliefs that might invite your opponent's criticism, and ensure you can adequately defend your own beliefs. Arrange your arguments in a logical way. If your arguments are not presented in a straightforward, understandable way, your message will be lost on the person you're talking to, and your arguments will be weak. For instance, when explaining how one's religion is culturally determined, you should get the other person to agree with each of your premises (the basic facts that lead to your conclusion). You might say, “Mexico was settled by a Catholic country, right?” When they answer yes, move on to the next premise, like “Most people in Mexico are Catholic, right?” When they answer yes, move on to your conclusion by saying, for instance, “The reason that most people believe in God in Mexico is the history of the religious culture there.” Be accommodating when discussing God's existence. Belief in God is a sensitive topic. Approach the debate as a conversation in which both you and your conversation partner have valid points. Speak in a friendly manner to your conversation partner. Ask them for reasons as to why they believe so strongly in their faith. Listen patiently to their reasons and tailor your responses appropriately and thoughtfully to what they have to say. Ask your conversation partner for resources (books or websites) that you can use to learn more about their perspective and beliefs. Belief in God is complicated, and statements about God's existence -- either for or against -- cannot be taken as fact. Stay calm. The existence of God can be an emotionally-charged topic. If you are excited or aggressive during the conversation, you might be incoherent and/or say something that you regret. Try deep breathing to stay calm. Breathe in slowly through your nose for five seconds, then breathe out through your mouth for three seconds. Repeat until you feel calm. Slow down your rate of speech so that you have more time to think about what you want to say and avoid saying something you later regret. If you start feeling angry, say to your conversation partner, “Let's agree to disagree,” then part ways from them. Be polite when discussing God. Remember a lot of people are sensitive about their religions. Be respectful of those who believe in God. Do not use offensive or accusatory language like bad, stupid, or crazy. Do not call your conversation partner names. In the end, rather than making a concise point, your opponent will often default to "I'm sorry you're going to Hell.” Do not respond with an equally passive-aggressive retort. 
Propose that living beings are poorly designed. Show the history of replacing supernatural with natural explanations. Demonstrate the inaccuracy of creationism. Argue that belief in God is socially determined. Explain that just because most people believe in God, it is not necessarily true. Explore the variety of religious belief. Demonstrate the contradictions within religious texts. Argue that if God exists, he would not allow so much unbelief. Explore inconsistency in the other person's belief. Explore the problem of evil. Demonstrate that morality does not require any religious belief. Demonstrate that a good life does not require God. Explain the contradiction between omniscience and free will. Show the impossibility of omnipotence. Put the ball in their court. Do your homework. Arrange your arguments in a logical way. Be accommodating when discussing God's existence. Stay calm. 