What IS good, and how do we determine it?

[Esquilax]

A presupposition is a thing which is implied or understood without being expressed at the beginning of a line of argument.

And from what did you draw this implication? If it's just that he didn't automatically agree with you on every point then you have a problem, because then you're either assuming that your arguments are inherently convincing, one hundred percent of the time, so that the only possible way a person could disagree is unfair bias, or you're just plucking the least flattering possible reason someone could disagree with you and flinging it at them for no reason. If your claim is "you just have a presupposition against my beliefs!" then you need more than "you wouldn't believe me!" as a reason. Unless you're actually saying there's no other possible reason not to believe you, in which case it's you that has the presupposition, not him.

Have you ever read his profile? He's not only an atheist but he self-describes as an anti-theist.

IOW, not only does he not believe in God but he is opposed to anyone who does.

Which still doesn't denote a presupposition, just a recognition of the harm that religion causes. There's nothing in being against religion, religious people, anything, that prevents him from accepting the factual accuracy of arguments for god; you can believe in a god and still be against him and his posse, after all.

That's taking things to a whole other level.

Beyond that, do you read his posts? He's not hesitant to share his beliefs.

So are you. Does that mean you have a presupposition too? Is that the only reason one might share their beliefs?

I think my assessment of Cato's presuppositions was reasonable in light of these things.

None of those things even hint at the existence of a presupposition.