The phrase "immutable truth" means a truth or fact that cannot be changed or altered. It is something that remains constant and unchangeable. Full definition
... The abiding and immutable truth of metaphysics and revealed religion must be reclad in new garments woven by a scientific and historical knowledge incomparably vaster than was ever before possessed by man.» (faith.org.uk)
If there's one immutable truth about how things are accomplished in Washington, it's that the squeaky wheel may not always get the grease, but the quiet wheel gets nothing. (suffolktimes.timesreview.com)
I'm not saying immutable truths don't exist, certainly they do. (entrepreneur.com)