There's an that involves religion and could apply to your case:

Sacred cow

from Wiki and Collins

"someone or something that has been accepted or respected for a long time and that people are afraid or unwilling to criticize or question" (emphasis, mine)

With "Holy" there's Holy Cow, but that does not apply to your definition; it is just an exclamation to avoid blasphemy –from a Christian centric point of view– though it might be blasphemous for an Hindu.

There's an idiom that involves religion and could apply to your case:

Sacred cow

from Wiki and Collins

"someone or something that has been accepted or respected for a long time and that people are afraid or unwilling to criticize or question" (emphasis, mine)

With "Holy" there's Holy Cow, but that does not apply to your definition; it is just an exclamation to avoid blasphemy –from a Christian centric point of view– though it might be blasphemous for an Hindu.