[Can we philosophically “prove” that God must be good?]
"The argument"
No philosophical 'argument' required. Just the facts, ma'am:
"If Atheism is true"
No 'If' about it. We who don't acknowledge the existence of a 'God(s)' - because there's no proof to our satisfaction that such exists - exist. Objective facts (such as we Atheists exist, and your 'God' doesn't) are superior to subjective 'Truth'. Deal with it.
"then all Atheists have absolute knowledge and can know everything (we know that this isn't true because of disagreement so this hinders Atheism, but for the sake of argument..)."
We Atheists, by definition, know infinitely more than you fundies. Knowing more. As a result of receiving a proper state, accredited, education. Results in one thinking for themselves. As opposed to fundie 'hoemskuling'. Ergo, our more than infinite superiority to you fundies. QED.
"If Atheists can know everything, then they are just as knowledgeable as what we define to be God."
We exist. Your 'God' doesn't. One word: science. PROTIP: According to your 'Word of God', Pi = exactly 3:
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The more scientifically-minded ancient Greeks (as in BCE) knew otherwise before said 'Word of God' even existed.
"This makes Atheists God."
Well, we are infinitely superior to him after all, so thanks for the compliment. I've never killed anyone in my life, nor do I want to, ever:
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"Therefore God is an Atheist."
He made me an Atheist. Who are you to question his will, hamashiachagape?
"YIKES! shoddy logic."
From the point of view of we Atheists, not really. But from your point of view, it is. You yourself have proven that we Atheists are infinitely superior to your 'God', after all.
I love the smell of annihilated arguments in the morning. Smells like... victory.