Well actually it is arguable that it is possible to know that nobody knows.
From a philosophical standpoint on the concept of knowledge and the value of subjective "knowledge" versus the objective.
Since there is no way of expressing or experiencing objective knowledge of God, and that the only experience people can have of God is personal religious experience which is highly subjective. It is quite arguable that there can be, philosophically, no "true knowledge" on such matters and that it is therefore impossible for anyone to "know".
Some atheists also argue that in this world of science that you can also know God does not exist. However anyone supporting this idea needs to learn more about the scientific method. Which in no way supports such a baseless claim. And baseless is what it is.
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I would also make the point that given religion is based on faith, the whole point is that they do not "know". Being a religious person and arguing that you do in fact "know" of God's existence would defeat the point of having faith would it not?
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No offense intended if given to anyone. I know people can be touchy on this stuff. I just have a philosophical interest on this subject, being a proud hard sceptic agnostic. I made a topic once on these very forums about my views on atheism... Got messy!