|The "three main reasons" why there is no god (i.e. why god does not exist), i.e. reasons for being a zerotheist (aka smart atheist), from American electrochemical engineer Libb Thims's "Zerotheism for Kids" (2015) lecture. |
See main: Disproofs of the existence of godThe three main proofs that god does not exist, are (a) that according to comparative religion and mythology (see: religio-mythology scholars), gods are all rescripts of astro-theological entities, e.g. stars, planets, constellations, galaxies (e.g Milky Way), the sun, rivers (e.g. Nile River), and (b) modern science has explained everything that god or god of the gap once represented.
The so-called “Dostoyevsky dilemma” (Ѻ), reads as follows:
“If god does not exist, everything is permissible.”— Fyodor Dostoyevsky (1880), The Brothers Karamazov
This gets into, among other things (see: physicochemical morality puzzles) the question of “legislating morality”, without god as backer. Frank Turek (2003), citing previous quote, puts it like this: 
“Nietzsche understood that when god ‘died’, all absolutes died with him. Therefore, with no ‘moral lawgiver’ to prescribe right and wrong, he claimed we must create our own morality. After all, if values do not originate in heaven, then we must make them up on earth.”
The following are related quotes:
“Meslier’s war cry [Testament, 1729], never before heard in the history of western thought, offers one of the first true atheist moments, if not the first. Prior to him, they call the agnostic an atheist who, as Protagoras, concludes that when it comes to god one can conclude nothing; the pantheist who, such as Spinoza, affirms its existence consubstantial with nature; the polytheist, like Epicurus, who teaches its multiplicity; the deist, in the way of Voltaire, for whom god creates the world en bloc, but does not care about the details; or whoever’s idol does not correspond to the strict criteria established by the church. Now, the atheist clearly says that ‘god does not exist’. This is what Meslier clearly writes: ‘there is no god’ (chapters 59, 74, 93, 94)—that is clear and distinct, blunt, and straightforward.”— Michael Onfray (2009), “Preface: The War Song of an Atheist Priest” (pg. 19) 
“Live a good life. If there are gods and they are just, then they will not care how devout you have been, but will welcome you based on the virtues you have lived by. If there are gods, but unjust, then you should not want to worship them. If there are no gods, then you will be gone, but will have lived a noble life that will live on in the memories of your loved ones.”— Marcus Aurelius (c.160AD), Meditations, Book II 
“Natural atheology, the attempt to prove that god does not exist or that at any rate it is unreasonable or irrational to believe that he does—in which the most widely accepted and impressive piece or representative has to do with the so-called problem of evil—is the opposite of natural theology, attempts to give successful arguments or proofs for the existence of God [in nature].”— Alvin Plantinga (1974), God, Freedom, and Evil 
“Yes, I do have proof that god does not exist. It is perfect and irrefutable. I’m not going to show it to you however. You cannot detect it in any way. You cannot deduct it from the laws of logic either. You might claim that I do in fact have no such proof, but you have not proof that I don’t. Sound familiar?”— Rune Friberg (c.2013)
● Atheism types by denial and belief
● Belief in the existence of god by scientists
● Existence of god
● God does not play dice
● God hypothesis
● God is dead
● God void issue
● God vs Gibbs
● Gods energy
● Soul does not exist / Asoulism
● Year god was disabused form science
1. Thims, Libb. (2015). “Zerotheism for Kids” (co-host: Thor) (main) (vid), 5-intro sides (Ѻ), 56-main sides (Ѻ), 3.5-hours, Chicago, Aug 10.
2. Aurelius, Marcus. (c.170). Mediations, Book II (Ѻ)(Ѻ). Publisher.
3. Plantinga, Alvin. (1974). God, Freedom, and Evil (natural atheology, pgs. 2-7). William B. Eerdmans Publishing Co.
4. Geisler, Norman and Turek, Frank. (2003). Legislating Morality: Is it Wise? Is it Legal? Is it Possible? (pg. 62). Wipf and Stock Publishers.
5. Onfray, Michael. (2009). “The War Song of an Atheist Priest”, in: Testament: Memoir of the Thoughts and Sentiments of Jean Meslier (translator: Michael Shreve) (Preface, pgs. 17-24; quote, pg. 19). Prometheus Books.