I completely agree it is problematic if not impossible if your goal is to use some sort of universal logic to persuade others that you are right and they are wrong. I don’t have a solution for that and as far as I can tell only religions claim to have an answer which is basically “our god is right and says X is right and Y is wrong.”

I’m not sure I would go so far as to say that means you are not “legitimate” for opposing harm to others. The question that springs to mind is, who gets to decide what is objectively legitimate? The question sort of presupposes there is some higher authority which can be appealed to (a god, etc). If you aren’t basing your reasoning in religion/gods then I don’t see how you can logically derive morality without relying on your personal beliefs. I’d love to see someone show me how they think morality can be objective without a religious premise.