What is Set In Stone?
1.
Colloquial description of an item (document, schedule, contract, etc.) that cannot be altered without great effort (and possibly some destructiveness) for one reason or another.
Probably derived from how the ten commandments (Exodus 22:* in your bible) were produced on stone tablets, as it requires effort to modify a document that is etched into stone.
See also
Once you sign the contract, the terms are set in stone.
