Over at Quora, I ran into a passel of atheists making claims like this. So I'm inviting them over here to hash it out. We may get some new members out of this.
My thesis is that strong atheism is easy and fun to defend, and perfectly honest.
I don't claim that it's the only honest position; I have nothing against weak atheists. (I do think it is tactical idiocy for strong and weak atheists to attack each other.)
Strong atheism is exactly as "faithy", dishonest, awkward and hard to defend as is the assertion that Santa Claus definitely does not exist.
Declaring certainty about the non-existence of the impossible is not a problem for me; I am as happy to assert that Gods do not exist as I am to assert that square circles and married bachelors do not exist.
Of course, like many arguments in the theological realm, it is possible to find loopholes by re-defining what qualifies as a deity. Emperor Hirohito existed, and is considered to have been a God; Joe Stalin existed, and is considered to have been a God too. Some theists define their Gods in such a way as to force them to be compatible with observation; Personally I regard such Gods as unworthy of the name, in the same way that a Cat is unworthy of being called a Dog, unless you bastardise the definition of the word 'Dog' with the explicit aim of including cats.
As a minimum requirement, I hold that an entity can only be considered to be a God if it can generate exceptions to physical law, or can routinely bias genuinely random events in favour of a goal. I am completely confident that no such entity is possible; and I am equally confident that any entity not meeting this definition falls outside the reasonable range of definitions for a God.
Of course, that minimum requirement allows us to imagine a very wide range of fictional Gods indeed - and as history shows, a very wide range of such Gods have been proposed. The existence of any of the sub-set of 'worshipful' Gods - those who deserve and/or reward the respect or worship of humans - is impossible for a considerable number of independent additional reasons, even if we were to allow the initial definition to be stretched even further, to the point of including hyper-advanced intelligences that work within physical law, but use technology so advanced as to inspire quotations from Arthur C Clarke.
In short, I am completely certain that none of the myriad entities ever proposed as Gods by anyone, actually exist outside their imaginations, except in those cases where their definition of the word 'God' includes entities (such as Hirohito, Caesar or Stalin) that have no supernatural qualities, in which case I reject their use of the word 'God' as simply erroneous.
The word God, like the word Werewolf, refers only to a fictional construct. It cannot be correctly used as a literal description of an actual entity. A pantheist might declare 'God' to be synonymous with 'Universe', and then say that as the universe exists, so therefore does God; But I reject this as the sophistry it clearly is. If we already have a name for an entity, then nothing is changed by using 'God' as a synonym, and the word can simply be discarded.