When person X says that “This (specific religion/belief) is true for me and that (specific religion/belief) is true for you,” what we then have is a subtle violation of the laws of logic.
Firstly, person X is essentially propagating an “all truth is relative statement.” However, the unavoidable conclusion is in this process person X is also making an absolute truth claim. This is an issue since one cannot hold to absolute truth as well as relative truth for the simple reason that they are opposites. So, if person X makes a relative truth claim then he is also unknowingly making an absolute truth claim, namely that all truth is relative. Thus his claim is self-refuting and therefore cannot be true. Secondly, if what is true for me is that relativism is false, then is it true that relativism is false? This destroys the notion as if one replies by saying no then what is true for me is not true and relativism is therefore false. If you ones says yes then relativism is false. Person X might then retort by asserting that such discussion is pointless because we can’t know anything for certain; so how then can anyone make a claim to know the truth? This is also self-defeating for the reason that saying we cannot know anything for certain is itself a certain truth claim. On his worldview person X must at least admit one truth: that we can’t know anything for sure. And if he does that then he’s worldview is rendered logically invalid.