Changing people's minds on anything is hard, maybe more so if you're firm on being honest rather than, say, a con artist who is trying to swindle them. But there are approaches that are sometimes more effective. For one thing, with something this messy and complicated, sometimes you need to start by working out what the person actually believes to begin with.

Like a small child asking "why" but with more nuance, so you don't just come across as annoying. Ex:

"Marxism has been tried and didn't work."

"Do you believe China is succeeding?"

"Somewhat, but they aren't really socialist."

"What do you believe defines a state as socialist in practice vs. not?"

Or

"No, they are failing and repressive."

"What have you seen or read that makes you believe that?"

You can also rephrase what they stated back to them, both to see if you're understanding correctly and to get them to think more about the words:

"Marxism has been tried and didn't work."

"So you believe Marxism has been thoroughly tried in detail, on an organized level, in the ways that Marxist theorists have laid out as how it should be done? If so, what does that look like to you?" (Here you're also putting it on them to explain what they think the practice of Marxism is in the first place, which may not be clear.)

This is not a shortcut to changing minds, but it can help investigate where someone is coming from and also encourage them to think about what they're saying, without you coming across as condescending and without having to get into long defenses of your views before you even know what they believe about them. It can also take patience and may be kind of annoying to do, depending on the situation, if you more just want to have a back and forth.