First off, I am not certain there is no god. But the complete lack of evidence supports the notion that any god is fictional. For your information quantisedd, an atheist is defined as a person who lacks a belief in a god or gods. That is not the same as one who denies the existence of a god.

So asking me how I feel knowing there is no god is like asking me how I feel knowing there is no Bugs Bunny.

But I wish to add to this.

I was raised in a christian family, left organized religion in my late teens, and spent the next forty years searching for anything spiritual. Eventually I came to the realization that there was no evidence for a god, and that I was an atheist. And for me, that day was one of liberation.

I was free from the fear imposed by religion. I was free to think for myself, and make my decisions based on better things than what was dictated over two thousand years ago by barbaric tribes. I was free to find a much better level of morality.

There has not been any negative effects, with the sole exception I lied to keep my atheism from my mother. But for every other aspect of my life, my outlook is incredibly positive, I relish each day, and enjoy the company of my friends even more.

And I face death without fear.

I did not intend to be an atheist, I did not seek out atheism. But my decisions were driven by logic and a skeptical outlook, and I arrived at a decision that has improved my life in every area.

Quantisedd, you asked, “Why do atheists feel that they are certain that there is no God?”

I am atheist. I do not feel certain there are no gods. However, if you’re asking about one in particular, please let us know, there are, after all, so very many.

Ah, the typical supernaturalist assumption that "atheists" are a homogeneous group akin to ants. How tiresome.

First of all, I'm interested in approaching this issue from a rigorous standpoint, and as a corollary, recognise that the question of the existence of a god type entity is unanswered. All the observational data available, informs anyone who pays attention thereto, of this elementary fact. If the question had been successfully answered in the past, there would be no more arguing about it - whatever data leading to the requisite conclusion would have been subject to scrutiny, passed every test thrown at it, and the issue would be settled.

Second, I don't assert that no god type entity exists. I leave the business of disseminating unsupported assertions to supernaturalists. Instead, what I recognise, on the basis of the relevant observational data, is that mythologies are incompetent to provide an answer to this question. Mythologies demonstrably contain assertions about the world that are not merely wrong, but fatuous and absurd - assertions that are manifestly the product, not of a fantastically gifted magic entity capable of fabricating a universe, but instead of semi-literate superstitious nomads.

Third, I also recognise another principle in operation here, namely, that the moment testable natural processes are demonstrated to be sufficient to explain a given class of entities and interactions, then supernatural entities (however these may be defined - I've yet to observe any consensus arising from supernaturalists with respect to a rigorous definition of 'supernatural') are rendered superfluous to requirements and irrelevant. Even an elementary perusal of the five million or more peer reviewed scientific papers extant in the literature, informs those performing this task, that this has already happened for vast classes of entities and interactions, and that the few remaining areas of active research are not yielding any data breaking this precedent.

Fourth, although the combination of mythological incompetence and scientific success rule out infantile imaginary magic men, they do not rule out entities of the following sort:

[1] Any god type entities that are consonant with, or provide consistent extensions to, the known laws of physics;

[2] Any god type entities that operate through the application of testable natural processes, including processes of this sort awaiting discovery.

However, at this point, the question has to be asked, why any such entity that does exist, has been apparently exerting much effort to conceal itself. Though this question, of course, only becomes important, if one presumes from the outset that any such entity would have any interest in us humans at all. An entity completely indifferent to us, would of course be in no hurry to reveal itself, and would, courtesy of operating via testable natural processes, be difficult to distinguish from testable natural processes arising without a god-type entity's activity.

Fifth, since I've already engaged here in several interesting speculative excursions on this matter, the mere fact that I have done so, and presented said expositions honestly as speculative excursions, should be telling you something important. An example thereof can be read in full here. Read and enjoy (or not, as the case may be).

Indeed, the one fact that time and again presents itself to me, is that the one group of people who routinely demonstrate paucity of thought on the matter of the "god question", are supernaturalists. Who routinely assume that the only possible candidate purportedly must be the one asserted to be thus in their favourite mythologies, regardless of how fatuous and absurd the assertions contained in said mythologies are. The mere fact that I've been able to conceive of a better god type entity than those presented in mythologies, courtesy of my own speculative excursions, should be informative here not only with respect to your presumption laden question, but to the likely outcome of any proper, rigorous investigation thereunto. See here for another example of brief musings on the matter.

But that's another aspect of the supernaturalist aetiology I've become familiar with over the past decade or so - namely, the indolence with respect to prior research that is endemic to supernaturalism and its adherents. Most of the specimens thereof who appear here, routinely demonstrate that they never bothered to ask themselves substantive questions, including questions about the validity of their own presuppositions about us and our thinking, many of said presuppositions invariably consisting of repeatedly destroyed canards that we can recognise as such in our sleep, and detect incoming from several light years away. Indeed, one of my own favourite rejoinders upon yet another instance thereof, is "It's petunias time again, folks", which raises smiles among those understanding the Douglas Adams reference.

@quantisedd: "Certain that there is no god?" No one has yet defined, explained, or demonstrated a god into existence. We have millions of failed gods. Belief is allocated to the degree of evidence provided. There is no more evidence for the Christian or Islamic version of god than there is for any other version of god on the planet. That does not mean "Atheists are CERTAIN there is no god." Most atheists are certainly certain that all the gods they have heard of are either useless contradictions, ineffective and therefore useless or simply non-existent. If you have a god that you think is real, we would all love to hear about it. Unlike the theist population, atheists are certainly open to changing their minds. All you need do is give us the information to do so.

Why do theist believe in any god that they can't prove is real? Why can't theist give objective evidence that any god is real? If any god was proven real, what makes you think it's your god?

Re: OP - "Why do atheists feel that they are certain that there is no God?"

(Before reading other responses...)

Yo, Quantisedd! Quite presumptuous, aren't you? Plus, you might want to define your particular god before asking such a ridiculous question. Just a suggestion. Anyway, for the sake of shits and giggles, I will pretend you are referring to the god of the bible. (Please correct me if I am wrong.) So, with that in mind, while I can not speak for other atheists, I CAN tell you that THIS atheist has never claimed to be certain of such a thing. I simply do not believe in that god. No reason for me to do so until somebody can somehow demonstrate that particular god is real. Just that simple. Period.

Oh, on a side note, by the way... *sniff-sniff-sniff*.... *gag*.... Umph... Uh, you might want to change your cologne. Eau de Gym Sock is not a very pleasant fragrance.

Edit to add: Whoops. Just checked your profile. You must be referring to your precious darling little Allah. Same goes for him, just so you know. (Of course, he's pretty much the same as the bible god, anyway... *shrugging shoulders*...)

Wait a minute...

I'm an atheist and I do not claim to know, with absolute certainty, that gods do not exist.

My position is ,that theists have never been able to meet their burden of proof to support their claim that a God exists.

And as long as theists continue to fail to meet their burden of proof, my atheism will continue.

My atheism is a provisional position, not a dogmatic one

Let me add, that I am open to being convinced that a god exists.

All that it would take to convince me is, demonstrable and falsifiable evidence, and valid and sound logic, to support the claim.

But without meeting the above criteria, why should I be warranted in believing a god exists?

I'll start by saying that when you say 'no God' I assume you really mean 'no god'. I generally refuse to say "God" because someone who capitalizes god never really means a god in general, they mean a very specific god, one of the various iterations of the Abrahamic god, which goes by the names Yahweh, Jehovah, or Allah, depending on your specific religion.

That said, I'm not absolutely certain that a god, or gods, do not exist. It's just that so far any evidence brought forth for a god, (any god, not just "God"), has never convinced me that said god actually exists. If there's no sufficient or compelling evidence that this god exists, then there's no real reason for me to believe in him/her/it.
I don't know for certain that Darth Vader or Sauron or Cerberus don't exist either. But again, I see no reason to believe that they do.

In response to the question at hand, "Why do atheists feel that they are certain that there is no God?" I can say that I am about 98% certain that there is no god. The other two percent is the hope that there is no god. That hope is not out of fear of facing some type of god when I finally buy the farm on this life, but it is out of the fear of having someone that is a cruel dictator overshadow my life and one day, my death.

Regarding my certainty that there is no god, I think that we have to realize all that we can do in today's world. We have sent probes to every planet and at least one official dwarf planet in our solar system. We have investigated Mars with terristeral machines (rovers) and looked at many of the numerous moons that orbit other planets in our own backyard. We have even looked at planets hundreds, thousands and even millions of lightyears from our little solar system and been able to predict the atmosphere composition that surrounds those planets. We have searched our own planet and found fossils that do connect us to the genealogy that comes before us. We have also been able to trace a great many extinct animals that once roamed the Earth. What's more is that we've been able to, with a pretty good degree of certainty, tell the age of the Earth along with the universe at large, seen how stars form and how they die. We have satellites that can look down on us from low Earth orbit and tell what just one of us is doing in our backyards along with the outfit that we’re wearing while doing it. Some of those satellites have infrared vision and could possibly tell what just one of us on Earth are doing in our own bedrooms.

From every atheist/creationist/religious debate that I've ever heard to date, they all kind of go the same way. The religious person asks, "can you explain" this or that and the atheist pulls out fact after fact, scientific theory after scientific theory and answers those questions. Then the atheist flips the script and asks the religious person, "can you explain" this or that and the religious person starts to back themselves into a corner to finally say, "well, you have to just accept this on faith." Faith is not fact.

Another aspect of this also speaks to me: Charles Mansion was prosecuted and imprisoned for what his followers did... More specifically, his role in what his followers did (Murder, Conspiracy) for which he spent his entire life in confinement until his death in November of 2017. This concept is not only a United States law thing, where we punish the leaders as well as the followers for crimes. After the second world war, numerous members of the third reich were charged, and most were subsequently executed for war crimes. So this is not a new concept. When we apply this same concept to the religious, we get excuses. Over the years, I've heard, "well, you can't blame god for what his followers do." Why not? Especially considering that one of the most violent books in the world is the foundation of this religion.

Numerous accounts of rape, abortion, murder, conspiracy, slavery, cruelty, deceit along with other a whole list of other crimes that would land most people either in prison for life or on death row, nearly all of it with god’s blessing, permission or from the direct actions of the god character. Keeping that in mind, is it really any big surprise that in today's world we find that priests can't keep their hands off of children and now it seems that protestant ministers are following the same path.

Now we can get preoccupied with numerous fields of science that disproves the entire notion of god, including where the christian god even came from, the legends and myths that are older than the christian version of god. But what about common sense and personal experience? What about the things that don't make it into scientific texts? What about the children and sometimes even adults that were raped, beaten, tortured and even sold into slavery? And no, I'm not talking specifically about the middle ages... I'm referring to the 80's (the last case of such that I am aware of in the United States, even though I freely admit that I've not gone looking for more recent cases of this.) What about those people that few ever hear from for one reason or another?

I submit that we should add all of this together, and even more to the long list of crimes against humanity that religion in general, christianity in particular has committed. I submit that we should, as a society, put every religious text where they go, in the library right next to mother goose because all of them are myth, fable, and fairy-tale and regard it as the dangerous thing that it truly is.

I'm yet to be convinced there is a divine being of any type, I think the descriptions of a possible god are pretty well thought out (classical theism ? ) but I'm yet to be convinced that the logic we use is useful in determining intelligence beyond our own.

But when someone approaches me and claims not only that there is a god, and what's more that same deity wants me to know him personally, wants to put a part of himself in my heart and then claims to have made everything in existence, who is all powerful and all knowing then I'm 99% certain that such a being does not exist.

I think such a deity would be able to introduce himself to me, in a way that isn't exactly the same as no deity !

Reading Karen Armstrong, and Bart Ehrman and listening to Sam Harris put to rest my shaky Christian belief. I say "shakey" because I was never a die-hard Christian, to begin with. My family went to church more out of a social obligation rather than spiritual guidance. The bible, in our house, was never consulted when it came to moral or ethical behaviors. We, my siblings and I, learned about right and wrong from our parents, our neighbors, and our teachers, especially the bad ones who gave us a living example of what it means to be mean or cruel.

It was not a difficult jump for me to question the existence of an omniscient deity and declare myself an agnostic. I tried to hold out for an all-powerful, creative deity, at least one who was superior to the Abrahamic god of the Hebrew nation, but even that attempt came to naught. What I have settled for is a blend of Zen Buddhism and Taoism. Hence the word "god" is synonymous with the word "devil." They are both human constructions as a means to power.

Yes, terrible things happen in our world and most of them are a direct result of human activity. Good things happen as well, not because a god intervened, but because moral and responsible humans intervened. For example, not long ago I read a story of a young girl who became lost on a hike and fell some thirty feet down a cliff and landed unconscious on a rocky ledge. Her parents prayed to God. She was found by skilled trackers with tracking dogs. Her position was relayed, by GPS, to a rescue base. A helicopter piloted by an excellent pilot hovered over the cliff while highly disciplined rescuers were lowered and managed to lift the girl to the helicopter where she was flown in haste to the nearest hospital and trained, intelligent surgeons operated and saved the girl's life. The parents declared God had worked a miracle. I was stunned. There was no God. If it had been up to a god, the girl would be a skeleton on that ledge. Good, intelligent, responsible humans who cared enough to do the job for which they had trained, saved that girl.

The only god is us. We who strive to learn and understand this incredible miracle of a planet and strive even harder to create and maintain a peaceful world deprived of mythic gods who purportedly tell us to murder each other for their sake.

Because it's bollocks.

An invisible pan-dimensional wizard, who creates the universe (not proven), created the earth (demonstrably wrong), created humans (empirically proven to be a horse shit claim) and all other of wonderful garbage to make the incredulous feel all warm and fuzzy.

If there was a god, why would there be zero evidence? Why would kids get bone cancer? Etc...

The facts are that we are superstitious moderately evolved primates, that seek patterns in nature and are incredibly superstitious.