Hmm, that is pretty vague. So, in this world view Homer would not be a thief because it was common and acceptable (even considered a compliment) to copy the works of others entirely in his time, but today copying is theft because... well, I guess saying that there is a big media campaign financed by corporations that have an interest in making money off media is slanting things a bit.
Today copying is theft because why, exactly?
Is all copying theft? Is repeating large parts of Monty Python dialog theft if you don't make money off of it? What if you are using it to impress a girl (good luck) or a potential employer (ditto)? Or in a High School drama activity that charges admission? Or a fundraiser for the local library? YouTube? As part of a comic sketch that is how you make your living?
You don't have to actually answer all those questions, just something to think about. I don't really know when you consider copying to be theft, if you don't consider law to be a valid reference point. As far as those examples above, I imagine if you asked 10 people you would get a half-dozen different answers, at least.
And if you asked what are "the right things for others, for society and for posterity -- in that order.", you are certainly going to get people who honestly believe in things that you find abhorrent.