I've found it best to avoid the word "truth" whenever possible. The concept of "truth" implies an objective reality exists and that you know about it. Since we may be in a simulation, in the imagination of a god, or just hallucinating, we can never really be sure about "truth" and I find it boring to play semantic games in order to better hedge the word.
I find it much better to just focus on predictions and beliefs with explicit levels of confidence.
If you're talking about whether the sun rises tomorrow, and you say you predict that it will rise with high confidence, and your interlocutor responds, "That's not my truth," then you can just ask them to break that down into a prediction. Are they saying the sun won't rise? If so, okay, you can test that.
If the disagreement is over something that can't practically be tested, you can still interrogate their concrete predictions and see where they disagree with yours.
Religious people love talking about Truth because it is so confusing. I can't nail you down and show where you're wrong if you refuse to be concrete, so if you don't want to be shown to be wrong, just talk about abstract Truth.
Since we may be in a simulation, in the imagination of a god
Even then, what the simulation is actually simulating or what the god is actually imagining would still be a matter of fact on which people could disagree but at least one of them would be wrong.
Can someone help me dissolve this, and give insight into how to proceed with someone who says this?
What are they saying, exactly? That the set of beliefs in their head that they use to make decisions is not the same set of beliefs that you use to make decisions?
Could I say something like "Yes, that's so, but how do you know that your truth matches what is in the real world? Is there some way to know that your truth isn't only true for you, and not actually true for everybody?"
I'm trying to get a feel for what they mean by "true" in this case, since it's obviously not "matching reality."