How to easily defeat any argument for God

[Belacqua]

Goodness and Badness of x, is more like saying the roundness or yellowness of X, or the light or darkness of X, than the disgustingness or prettiness of x, or the taste of x, or the feeling of x.

I'm pondering how to get past the current stuckness of this conversation.....

Maybe we could address some other traditional questions about the Good, and let people hate those instead. 

For example, do a good medicine, a good painting, and a good person, have some quality in common? Is the goodness of each of these something they share, or is each good in its own way? 

I think that Plato and Dante would both say that each of these things "participates" in the Good in its own way, reflecting or embodying some aspect of the entire Good. And pointing us eventually to the highest Good. 

Probably your opponents here will disagree with that. 

I haven't worked on this much, but I think we can make a case that each of those things does share a mutual type of goodness, if we define goodness as that which encourages human thriving. A good medicine obviously makes people healthier, and health is better for thriving. A good painting, in this view, would be one which enriches the lives of those who see it -- challenging, surprising, delighting. The painting's goodness would be less a practical goodness, nearer to autotelic, but is still good because of the affect it has on human life. A good person is good in relation to others -- bonum est diffusivum sui. It would be hard to say that a person who was only good to himself was very good. 

So given all that, I do think that the goodness would be a quality in the object. It would often be perceptible to us. Of course there would be disagreement and mistakes -- a medicine might prove to have unknown side effects, for example. A movie which at first strikes us as good might tend overall to flatter its audience and make people stupider. (A lot of them do.) 

Though your interlocutors here continue to think of God decreeing the Good like laws, theologians would say that God is the sum total of all the good aspects of all the things in the world (plus infinity). This is what Dante says. So you are correct, I think, to say that God couldn't reverse goodness and declare by fiat that the Holocaust was actually good. Because there is no possible world in which genocide encourages human thriving.