Trick question. You mean in the same way Roman Catholicism claims ex cathedra declarations of whomever is the current Pope at the time are said to be infallible? Who says that everyone must see that as acceptable or valid? What matters - and what everyone should be looking at - is are ones theological positions the TRUTH? Truth is not relative but absolute and is found, theologically speaking, in God's own infallible word. It doesn't change and remains true regardless if someone believes it or not.
So what do you measure someone’s theological position against to determine if it is true?