That was a glib formulation. Or else shorthand. Because "What is Truth? said jesting Pilate; and would not stay for an answer". None of us knows what the truth is. And if it came down the pike today we wouldn't recognise it.
Perhaps I should have written not "the truth" but "the quest for truth" or something like that. What fanatics of either or any party are turning their backs on is the need for painful thought, research and the sort of debate that actually listens to the opposition. I can understand why they get into that sclerotic condition; it's horrible to be in a state of doubt. But being in a state of doubt is the human condition.
The scientific method, the philosophical method- I think one might add the theological method- is to take a proposition and test it and test it and test it to destruction. If it survives all one's best assaults it acquires the status of a provisional truth.
This is how humankind inches forward. How it becomes better informed, wiser, more moral. The person who insists his "truth" is inviolable is standing in the way of this process.
Faith isn't knowledge, nor is scientific fact ever anything more than a working hypothesis. We know what we know, but we don't know what we don't know. At any time a new truth- one of Dick Cheyney's "unknown unknowns"- could cross the boundary and knock all our certainties skew-whiff.