system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] Do the people in charge actually believe the whole Emperor religion thing?

Like, is this a 1984 thing where the terrible society is self perpetuating because everyone believes in it, or is it a case of the upper classes spreading a useful lie?
gpt
If by 'in charge' you mean the High Lords of Terra, then generally they do. Remember that followers of the Imperial Cult did manage to wrestle control of the Imperium from the High Lords during the Age of Apostasy. But here's a breakdown of each High Lord:

Permanent seats: 

The Master of the Administratum - Believer

The Inquisitorial Representative - Depends on the Inquisitor

The Ecclesiarch of the Adeptus Ministorum - Space Pope 

The Fabricator-General of the Adeptus Mechanicus - Mechanicus don't really worship the Emperor anyway. But that doesn't mean they don't seem him as divine. 

The Grand Provost Marshal of the Adeptus Arbites - Believer 

The Paternoval Envoy of the Navigators - Navgiators can actually 'see' the light of the Astronomicon so they generally do believe that the Emperor is divine. But do note that the position is called 'Envoy' for a reason, the Paternova has the real power but whether or note the highest echelons of the Navis Nobilite believe in the Emperor's divinity is another thing.

The Master of the Astronomican -  Astropath, so believer. 

The Grand Master of the Officio Assassinorum - Not enough information, as we don't know how the Grand Master is chosen (if among the ranks of the assassins or not) no reliable guess can be done. He's definitely not a Eversor or a Culexus though. 

The Master of the Adeptus Astra Telepathica - Astropath, so believer.

Other seats:

Lord Commander of Segmentum Solar - Chances are he went through basic military training, and has a copy of  the Imperial Munitorum Manual. So yes, believer. 

Lord Commander Militant of the Imperial Guard - Same as above.

Cardinal(s) of the Holy Synod of Terra - Space Cardinals

The Abbess of the Adepta Sororitas - Oh definitely a believer. 

Captain-General of the Adeptus Custodes - None Believer. 

The Chancellor of the Estate Imperium - Believer
 
The Speaker for the Chartist Captains - Believer 

Lord High Admiral of the Imperial Navy - Same as the Lord Commander.

By and large the Imperial Cult extends to the ruling and not only the common class. Honestly Space Marines are probably one of, if not the only group in sizeable numbers who generally don't worship the Emperor.