A truth, or Truth? So many religious people see Truth, as a propositional FACT. That is not the case. That "cornerstone" Truth, is best understood in terms like the "Groundless Ground". The Center, which is everywhere, and no "where" at once. Don't mistake "a truth" (as you put it), with Truth, from which all truths are but forms. You cannot know that Truth propositionally. You know it in your Being, as that "Groundless Ground".A contradiction....
A truth is as firm as the cornerstone.....it won't be moved.
"A truth" is dualistic. Truth, is nondual.