The Emperor may not have been holy, but wasn't the Empire itself holy?
Oxford English Dictionary:
Originally Posted by Eusebio
Inspiring mingled reverence and admiration; impressing the emotions or imagination as magnificent; majestic, stately, sublime, solemnly grand; venerable, revered.
Deep respect, veneration, or admiration for someone or something, esp. a person or thing regarded as sacred or holy.
The actual word usually translated as 'Holy' when talking about the HRE was 'sacrum', which yes, applied to the Empire not the Emperor.
Originally Posted by Amallric
Latin: Imperium Romanum Sacrum
'Sacrum' means 'sacred' in the sense of being set apart for God: consecrated, dedicated to a religious purpose. It thus has a different implication than 'augustus' which means ''high and worthy of respect, as if divine'. Of course the Romans who gave that title to Octavian were pagans, and emperor-worship was a thing; the later Christian Romans were probably uncomfortable at the idea that one of the Emperor's titles originally had a religious connotation, so it was minimised.
Imperium Romanum means "Roman Empire" tbh, Empire of the Romans is Imperium Romanorum.