It's not, but you can't be just "agnostic" by itself.
You can say you're agnostic because "We don't really know" but you can't just be that, you either believe God exists or you don't. You can be Agnostic-Atheist or agnostic-theist but not agnostic on its own.
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So if we act like there isn't a god, does that make us Agnostic-atheists?
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If you don't actively follow a religion then you are an atheist. If asked if we will ever know if there is a god and your answer is idk, then you are agnostic. If both then you are an agnostic atheist.
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What if it's, "I'm not sure if there is a god or not." Why cannot that be an answer? "Maybe there is and maybe there isn't. We can't possibly know, and while it's infinitely fun to theorize, we can never truly say."
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To say ""Maybe there is and maybe there isn't. We can't possibly know, and while it's infinitely fun to theorize, we can never truly say."" is being agnostic, but in the end you still personally belief in a God or you don't.
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If you aren't actively following a religion then you are an atheist.